New plumbing shower has "beer flies"?

I had a complete re-do of a 1st floor bathroom. The bathtub was removed completely and replaced with a shower. The tile/plumber contractor created a new drain pan and wrapped it in rubber sheeting, up to the wall board, and then made a cut out for the drain, then poured concrete and made a p-trap of pvc below. Ever since I have noticed small flies like beer flies around the drain. I have flooded the drain with bleach, but the flies return on the walls and on the shower curtain. Anybody know what's causing them???