Flies in the house?

Two days ago or so, I noticed a lot of flies in my house. I thought that maybe they had flown in when my mother had had the back door ajar for around a half hour. Just a fluke. But ever since, I keep seeing more. I think there have been at least twenty, I just found four more this morning, and I feel like they are getting into the house some how. Doors and windows are usually shut and haven’t been open that much in the past few days because we have central air and it has been raining. Also, when they are open, we have screen doors and screen windows. Do flies nest like bees and could they have it connected to the house? I know I probably will have to call an exterminator if this problem persists but is there any way for me to find the nest or hive or is there no such thing? Is there anything I can do to find out the cause of this problem?