Did it just start happening? It could be the early stages of a urinary tract infection? Also I believe the frequent need to urinate can be a symptom of type 2 diabetes? That's probably not likely if you aren't having any other symptoms but it doesn't hurt to go to the doctor if it's a sudden change. If you feel fine other then peeing all the time I wouldn't worry too much about it.
PS I sometimes need to pee frequently too, though it's kinda random. Though like iconised ghost I pee tons when I have caffeine. Gimme some Earl Grey and I pee 3 times in an hour at least! :S