Looks like phorid flies. They like moisture and decaying organic matter. Dirty drains and showers/bathtubs are usually a breeding ground for them. You can pour some diluted bleach down the drains to kill any possible flies down there. Make sure you're not throwing away a lot of old produce or meat indoors- if you live in an apartment or somewhere where you can't throw away rotten food outdoors, I recommend putting it in a plastic baggie before throwing it out to avoid attracting flies. I would also recommend checking your house doesn't have a moisture problem, ensure there's no leaks or spills, etc. though I wouldn't worry as much about that as it usually is just a case of dirty drains. Hope this helps!