Help.. back pain while side sleeping

I am a back sleeper, but trying to transition to side sleeping for the growing baby. Since I started trying to side sleep, my lower back has been hurting pretty bad. I wake up multiple times in the night due to lower back pain and I have been in pain during the day due to the sleep position. The only relief I get is laying on my back. I have slept with a pillow between my legs to try to keep my hips aligned, but still no relief. Any suggestions on how to alleviate this pain? Thanks!