So I laid down in bed on my left side and all of sudden I'm having horrible back pain I'm 35 weeks…

So I laid down in bed on my left side and all of sudden I'm having horrible back pain I'm 35 weeks on Thursday. I haven't had Braxton hicks at all so I don't know if it's that or not. My abdomen isn't getting tight could it be Braxton hicks or her moving to birthing position?? This is my first so i have no idea. She's moving around fine. The pain is on and off.