I understand you not wanting to go to the doctor since you don't have insurance, but I'm afraid there's no way of knowing what happened to your back without seeing your doctor.
If you can manage to pay for one appointment, be sure to ask your doctor's office if he gives a discount for paying cash. Many doctors do. If you need further treatment, perhaps your doctor can suggest options to help you out. If worse comes to worse, you may have to go to the hospital emergency room. They are required to treat you whether or not you have insurance.