Low back pain while sleeping

I'm 16+4 with twins so I've already got a pretty big bump. This is my first pregnancy. Lately I've been falling asleep alright but then I wake up in the middle of the night with low back pain. I can't get comfortable and get back to sleep. I can't do this for another 5 months! I am already using a straight full-length body pillow. I usually sleep on my right side and hug it and have it between my knees. Lately I've been using a regular pillow behind my low back or sometimes under my bump. None of this is preventing the low back pain. I think sleeping in my back would feel good but I know I'm not supposed to. I wanted to avoid buying a $50+ pregnancy pillow but if that'll help I'm willing to do it. Other suggestions?