Feeling worried

I’m 17 weeks exactly. Whenever I lay on my back or switch positions, it feels painful due what I’m guessing is the gravity of it. It feels like the baby is sinking down and then my spine or my pelvis or whatever bones are behind there just pops. The pop isn’t painful but the pressure of the baby weighing down is. I don’t lay on my back often because it’s not recommended so it’s mostly just when I switch to laying on one side to the other. It’s like the in between when I’m facing up before I get to be on my side. The pop and the pain. I don’t know if what I’m saying makes sense but it is truly worrying me and I can’t go to the doctor yet because I have to get my medical release forms from my old doctor to give to my new.