When to call it quits

Im 30 weeks 2nd pregnancy and today im having extreme back pain. I cant lay sit or stand up all the way. Its even really hard for me to walk. I called the doctors and pretty much said they cant do anything. Ive tried a bath that didnt help now im laying with a heating pad and its not helping either. Its just a constant ache in my lower back. When should I call suffering quits and go to the ER? 😩