Contracting when I urinate...

Ok ladies, this may sound like a dumb question. Especially since this is my 4th pregnancy, although all my pregnancies have been different I don't ever remember this happening. It could also be because all my babies are 4-6 years apart... Anyway, EVERY time I go to the restroom to urinate my stomach contracts and its pretty uncomfortable and painful to get off the toilet. Have any of you ever experienced this? Is it normal? Btw, I am 34 weeks today and I do have a history of preterm labor (last two pregnancies)