Pregnancy back pain!

Kat • Pregnant with baby #3
I'm 17 weeks and I've had really had back pain from the beginning! Certain ways I lay or if I stand up for too long triggers it. Any suggestions on how to help it? Yoga positions? Exercises? Anything! Also, I know I shouldn't be but I'm the most comfortable at night sleeping on my back! Do you think the back pain is from that? Or from previous pregnancy?