I've got SPD/PGP but this week have also developed serious pain in my 'hips and buttocks when in bed made worse by of course lying on one side or another. I'm assuming its because the oval that is your pelvis is now all flexible and is contorting from lying on it.
The other night something really went and I was in agony and couldn't move. Did some googling and realised my piriformis muscle (didn't know I had one of those) had gone into spasm, which also explained the sciatica I'd been getting as your sciatic nerve runs through this muscle. It's the muscle that connects your coccyx with your femur running horizontally across your buttock.
Anyway, I'm doing the piriformis stretch 3 times a day and although it hasn't fixed the underlying pelvic problem it has brought my arse out of spasm..... So I thought I'd share it with you in case anyone else was suffering from hip pain in bed.
Lying down
Seated