This happend to me my last pregnancy. For some reason I felt like my daughter was going to fall out starting at 5 months. Didn't happen with my first. She was lower though. I ended up being fine, no preterm labor or anything.
It may be nothing, but you should def ask your doc about it. I think it may be a sign of preterm labor, BUT it could also mean that you're just carrying low. In any case, it's always best to check with your doc, better safe than sorry.