Have you had cold weather? The lower leaves on some of my plants took a hit with recent cold temps in the low 40s.

https://plantclinic.tamu.edu/2011/05/10/tomato-woes/

I'm thinking that it looks like sunscald.

Agree, a little sun damage, not to worry.

Ok, thanks.

So how could i recognize whether it's sun burn or cold damage?
I toughen them outside for some hours in the past few days but they are inside most of the time. When they're outside, the temperatures arent lower than 55F. (From around 70F inside).
It's also possible to be sunburn, as they are regularly exposed to direct sunlight (through window).

Could it be spider mites or thrips? Havent looked it up today, but it looks familiar.

Thrips

Cold damage looks somewhat similar but different.

My tomato leaves with thrips.