Huge fly problem....?

SO yesterday after work i come home and go into my daughters future nursery. There were like 30 small flies glues to the window. I never saw them in there before. They werent large, but they werent fruit flies either. I was scared they might be mosquitos but they werent long enough to be. Anyway, we just moved into this house recently and the room is small, but we keep the door closed to keep the dog out of the baby's stuff. Ill be having my child in August, and i dont want these things to keep coming back. Its disgusting, i opened the door to outside and swatted them all out with the broom, then came back later and saw there was either more of them, or they never flew out. They were there this morning as well. Theres no food, feces or water in the room but it is hot in there. How did they get in? Why are there so many?? Could they be coming from the attic through the vent??