Cramps at 18 weeks

So I took my boys to the park today (aged 4 & 6) and I pushed them on the heavy swing together and spun the roundabout fast, they had a great time. Only problem is now I have painful cramps in my bladder, like I'm due on, cramps in my lower back and a pressure pushing down.  I think I've overdone it. Just wondered if I should be concerned? Anyone else had this? I know it's not ligament pain as I have had that and it's different. No bleeding. I'm 18 weeks pregnant.