Where in the house is it located? How OLD is the house? Is it on wood or on concrete?

Have you pulled up a corner to see if there are indications of moisture/mold anywhere?

This is in my baby's nursery 😭 it's the middle bedroom, it has 2 bathrooms right on the other side of the wall on both sides. My house was built in 1977, but it's worth noting that my grandparents weren't very sanitary and didn't maintain the property well. I have not checked underneath, how do I do that? I don't want to accidentally ruin it

Oh and there's concrete subfloor underneath

Oh dear...I think I found the problem. The 1977n slab was NOT sealed like they are today. In fact the use of moisture barriers used underneath the slabs didn't start until the late 1980s or 90s.

The moisture in the ground MIGHT be wicking up from below (like when you sip soda through a straw...). As the moisture moves up through the slab, the top of the concrete is evaporating off the water. That creates a vacuum in the slab and draws MORE moisture up into the slab. And so on and so on.

The fall is a 'wet' weather time of the year (so is spring). I'm going to guess there was OLD carpet in the room? And when you pulled it up the pad (if there was any) had pretty much crumbled to 'nothing', am I right? That's a good indication that the moisture was moving EASILY through the old carpet.

New carpets, especially the PADS are great at blocking moisture. That means all the wet stuff in the concrete has nowhere to go. So it sits under the carpet pad and that's when mold can grow. Mold needs dark, damp with a bit of food (old glue, dander, dust, etc). When all three things are present you get moldy/musty smell.

If you installed it 5 months ago, that makes the MIDDLE of the summer - the dry time of the year (the earth under your house is as dry as it ever will be). Now in the fall the moisture is building up again.

Carpet is hard to damage when pulling up a corner. If you are unsure, you can always have the company who installed it come out and have a look. If they have moisture meters you might ask them to bring those along (offer to pay) so that a meter reading can be taken.

YIKES. So, yes there was old carpet.. and the carpet padding was WET and crumbled. We pulled out the old and left the bare subfloor for 2 years (we weren't living at the house yet, my dad was until he passed away) before just now installing this new stuff. No sealing or anything. And there was a slab leak left ignored for 2 years give or take... Sooooooo thanks you figured that out fast 😵omg now what do I do!?!

I would be very careful using baking soda on the rugs, I use to do that until the motor got so clogged with the soda I had to buy a new vacuum.

Oh crap, 😵 can I vacuum my motor with another vacuum?? I think I may have done that, but my vac is brand new so I was hoping it's not too late. I noticed the suction is not working as good so I quit using it for

Omg MAJOR PLOT TWIST... Sorry I forgot to reply before but I had scheduled a foundation repair company to inspect my foundation because all signs point to foundation issues all around the house, but he told me my lowest point is half an inch.... Which is well within being acceptable???? I pulled up the corners of the carpet, on the new carpet I was complaining about, there's no mold or anything underneath it but I'm still going to properly seal the concrete subs floor and have the carpet guy reinstall the carpet (he said that's actually no problem at all) but the carpet in my bedroom does have mold on it. There was nothing I could see underneath, but the black spots are all towards the middle and I would have to rip up almost the entire carpet to see. But I'm pretty sure that those stains are mold, especially because my bedroom carpet predates me, and my dad had a bunch of drunk domestic violence gross people who never once even thought about vacuuming and spilled a lot of beer on that carpet. But I'm still baffled, is it possible that the mold spores are being transferred through my HVAC system from one side of the house to the other and landing on my son's room carpet and causing the smell? I think it's important to note that the smell does go away after I vacuum but only for a short period of time and then it's back... Do you think there could be mold somewhere else that I don't know about? it's also worth noting that I am extremely allergic to mold and my asthma has been acting up really bad, to the point where now it's starting to negatively affect my everyday life. But I'm also allergic to a bunch of other stuff so it doesn't mean it is mold... Just throwing that out there

Your hunches are correct. The new carpet may not have enough damp stuff visible. The place where I would look = drywall. Damp drywall becomes rotted. It becomes VERY weak. It is very possible the bottom of the walls (where they come close to touching the floor) can wick moisture upwards. These panels are easy to damage if they are compromised. A flat head screw driver poked into the drywall will tell you plenty. If you can push the head into the drywall EASILY it is rotten.

Regular, strong drywall will NOT have issues with a screw driver head being poked into it. It won't do a darn thing. A damp/moldy bit of drywall will be easily pierced when poked at.

You have foundation issues. This is where you put ALL your money. Fix the issue with the slab. Rip out the carpet in your room. Have the slab scrubbed by a remediation company (or do it your self with the right concentration of bleach OR ammonia (please don't mix them).

And please do NOT reinstall the stinky carpet. If it is 'infected' with mold then it is infected with mold. There's no use reinstalling it unless you can be 100% sure the carpet can be CLEANED to the point that NOTHING survived the cleaning (fire kills mold as well as an ammonia bath or a bleach bath...but the CARPET will not survive either treatments).

Post photos of what you are working with. And when trying to figure out mold, look UP. Water always runs DOWN. Roofs/shingles/eaves/windows in bad repair will ALL let moisture in.

If this house is making you ill, it is probably best to sell.

Please get all old carpet out of the house. Your little one is breathing all that dust and nasties in the carpet. Also, new carpet can trigger breathing difficulties. You could always buy indoor/outdoor carpeting to cover floors until you find what you really want. Relatively inexpensive until you figure out what to do. My house was built in 1951 so I am very understanding of those concrete slabs.