33 weeks feeling cramps

Has anyone experienced something similar to this: 
I'm 33 weeks pregnant with my first baby, and lately I feel cramps that last for a long time just after I pee. 
Today they were so strong that I had to go the the ER to get checked and it turned out to be Braxton hicks. 
But the pain returns after I empty my bladder. Does anyone else feel this too? 
I'm still a bit worried!!