I wish I had an answer for you! I get this quite often (36 weeks tomorrow). I think it is the baby getting ready and everything kind of expanding and preparing since my baby is lower now. If it continues too long I'd call your doctor. I had a friend who didn't know she was in labor until she went in for her routine appointment. Good luck!
Have you had any loss downstairs?? Like a mucus or discharge?
Could be signs of early labour x