Since I Gave Birth To My Second Child (5months Now)

Since I gave birth to my second child (5months now) I have to put a pillow under or between my legs to sleep. If i dont use a pillow I roll onto my stomach and wake unable to move because no matter which way I try roll the back pain is unbearable. My husband has to physically roll me over to be able to move. Any suggestions as to what may be the problem?