Uterine irritability

Has anyone else experienced uterine irritability, I called my dr yesterday talking about abdominal tightening and had to go to the hospital to be checked for a bladder infection and or early labor. But it just turned out the be uterine irritability, I don't totally understand what that means. The dr said that basically the baby is just sitting on my bladder because there's no where else for her to go in my tiny tummy, causing me to pee a lot more frequently than I probably should. Has anyone else had this happen?