You need to go to the doctor and get this checked as soon as you can. The fact that it started so suddenly and the pain is so severe, that sounds like it might be something other than a muscle problem. This could be due to a gallbladder or kidney stone or infection. Ask for an abdominal ultrasound, a urine test, and blood test to check for infection. Do you have any lung symptoms (even mild), like shortness of breath, cough, tightness in your chest, etc? If so, ask for a chest x-ray or a chest ct to check for pneumonia, which could be causing inflammation in the lining of your lungs (I've had it, it's "unbearable").
If it is at all possible you could be pregnant, rarely this kind of pain (especially if it is in the lower back) can indicate a tubal (ectopic pregnancy). Ask for a pregnancy test, too.
If everything comes out clear, then it may be a muscle issue, but make sure they consider these other possibilities first. If they dismiss it as muscle pain and it's your gallbladder or something, that could be serious. Let us know what you find out. If the pain gets too severe or you can't get into your doctor, go to the emergency room or an urgent care clinic. Don't just grin and bear it.
