If there were any vents/flues to the outside that weren't properly sealed, its possible that the little b_ggers are getting in through there. But I think there are other, more likely explanations.

If you have gas appliances have the lines leak tested TODAY. Gas can collect,ignite and level the house and those next door. Call gas supplier before calling a plumber.

Are your windows and window screens sealed properly? I can't open mine anymore because that's when I get fiies. You may need to get the screens fixed.

Make sure garbage cans are sealed tight make sure there are no dead animals somwhere those are the 2 most common causes.

Are you able to go into the attic and check for flies?

Put fly trap jars outside to get rid of flies. Will be fewer flies to enter a home. Pets will attract flies.

Can ellie31 post a coherent post explaining what is going on?

It sounds like cluster flies to me. My experience was similar to that of raee in Ohio described above. The flies congregated along a window ledge (attracted to the light, I think) and I was able to vacuum most of them up with my vacuum cleaner because they were not real active fliers.

Nester44 I used my little battery powered vac to capture a bunch then laid vac on back porch. Later when I went to dump flies in trash ,they had flown away with the vac.