It could be a variety of problems, but one serious problem I can imagine is a kidney infection. Without knowing anything else about you but a complaint of pain, I cannot rule this out, which is why it is never a good idea to depend on some stranger on the internet to diagnose an acute problem that presents with pain. Acute pain is a warning signal, and should be heeded.
If this continues, and especially if you have a fever, see a doctor soon.
Kidney stones. Go to the hospital for a cat scan