Where could a bunch of flies in my house be coming from?

Last night I had about 10 flies in my room. I killed most of them, and looked around the house to see what was going on. I changed the cat litter, took the garbage out. I have no open food, except fruit. My cats’ food is in a closed plastic container. I have screens on the windows. I have no garbage lying around in the kitchen and my house is fairly clean, it’s cleaned every two weeks by a professional.

Tonight there are at least 6 flies in here. I can’t figure out why there are this many flies in the house. Can you help me figure it out?