Does the pain move? Is it sharp or tearing? Both sides or just one? Does the pain radiate around to the front at all?
My best guess is kidney stones, but answer my questions and I will try to help you out...
I would seek medical advice if this did not clear up within the next couple of days. A pulled muscle will take anywhere from 2 to 6 days to subside. I hope you feel better soon.
I have that right now! It tightens up like a mofo when I'm sitting in a stationary chair. I figured out it was the Tren so I stopped it and the pain is subsiding over the last three days.