Mine does this as well, and so did dd a ton.
Pelvic rocking/tilts are the only thing that helps. Or sometimes I just put my face to the floor/bed with my bum up in the air. My suggestion is to make sure you do your tilts in the morning (25x) and before each meal (25x) then 40x at bedtime. Go pee before hand and lay directly down after you do them so that the baby doesn't go back downward. Also, do 25-40 every time baby starts doing this, and and before and after any kind of exercise.
Hope this helps!
Holly, eternally in love with Kolby, Raising Juelie Anise (10y), Behnjamin Shen (6y), and Coen Syaoran (4y). Expecting June 2013