Starchild is right. Too much watering. People very commonly do this to seedlings. Try to only water it about once every two days or so with what you're doing. Everytime you water just keep spraying on top of the soil until water starts to run out the bottom of the cup if you made drainage holes. If you didn't make drainage holes take a lit cigarette and burn a few holes in the bottom of the styrophoam cup (it burns real quick but should stop at the soil.) Goodluck.