What you are seeing are probably filter flies, also known as moth flies or drain flies. They breed in decomposing organic material, garbage, rain barrels, around or under kitchen and bathroom sinks, and in water traps in plumbing fixtures. You should thoroughly clean your sink and bathtub drains to remove any gelatinous matter which might be coating the insides of the pipes. Any of the commercial drain cleaners will work well, or you can flush the pipes with hot water and bleach or vinegar. 

 Additional Resource 

http://edis.ifas.ufl.edu/ig091