How to know when a low dull back pain is preterm labor?

It could be, or you could have a bladder/kidney infection, or it could be that you pulled a muscle.. It's ALWAYS better to go in and get checked anytime your pregnant. Take some tylenol, try a warm bath and if you are still in a lot of pain I wouldn't hesitate for a minute to go in. Call a family member or friend, call a cab if you have to. If you are in preterm labor the hospital can give you medication to try to stop it. If is something else, (like a bladder infection) they would more than likely treat that with antibiotics. Good luck!