Some times yeast infections don't really carry any symptoms. You could simply be doing be over doing it with the masturbation....you could be too 'dry' down there doing it which can cause irritation. You could have caught something from the local pool, but not very likely since I would assume it's being cleaned regularly so that really shouldn't be an issue. Try not masturbating for a week or so and see if it improves, if not go see your doctor.
It may be from the amount of masturbation you do, really depending on how much you do it. It could just be an irritation.