contractions when needing to pee.

title pretty much says it all. I'll have a contraction and feel like I'm going to pee myself lol. once it ends I hurry up and go to the bathroom and I wont have another one for a few more hours. but same thing. I'm guessing its when I have to pee but cant tell my bladder is full? anyone else having this problem, and sometimes they hurt so much I just sit or lay there until they stop because I feel like my muscles are ripping apart. lol. . other wise I'm just having constant menstrual like cramps.