I am nearly 22weeks and have felt this for a while now. I always go to bed and wake up with period like back ache?
So I'm at 28 weeks, going into 29th. I have cramped and had back ache the entire pregnancy. My midwife assured me yesterday the cramps are round ligament pain cause by the baby and uterus growing. And the back aches are from the baby pressing on nerves. Sleeping with a pillow between your legs and one under your stomach relieves some of that. Also, a warm bath before bed can help relax some muscle tension. Good luck!