Feeling of "dropping" 20 weeks

Lauren • Loving this blessed life with my husband and baby girl! Can't wait to see what more God will do with our lives!
I'm a huge fitness fanatic and was training for a half marathon up until the end of March. I had been running 8-10 miles on the weekends. I had to stop because of a cervix issue. I continued to power walk and lift outside of running. 
Recently, I started running again. I started off at just a mile and built up. I ran a 5k on Saturday and everything went great. We were walking at the mall later on and it felt like something suddenly dropped down onto my bladder or something. I don't really know how to describe it, but it felt really weird. It went away quickly and I got the feeling once more on Sunday. Anybody know what it could be?