Urge to urinate- sign of pregnancy or something else?

I'm not going out of my way to fall pregnant as I have a 4 year old and 15 month old twins but we haven't been entirely careful as we haven't ruled out a 4th. Ever since ovulation I have had cramping almost every day like period pain and when I need to pee it comes on suddenly and I have to run! 
I don't have any burning or anything like that so I'm not sure what's going on. Any ideas?