K.D.
I had a similar issue with my both of my pregnancies....but it typically occured when I either A) didn't drink enough water or B) accidently laid on my back while later in pregnancy. Be sure to get enough fluids...I learned never to be without a bottle of water...and be sure to eat small meals on a frequent basis. It is a good idea to speak with your doc....in case something else is going on.
Good luck!