If it's when you move, it could definitely be a muscle spasm. But whenever I get a UTI my back can hurt (where your kidneys are). Just pay close attention when you pee, if it hurts or is really dark colored you might want to make sure your not getting a UTI.
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Re: It feels like someone is stabbing me in the back...
Sounds like a muscle spasm or strain...like maybe you slept on it funny or twisted wierd?
It might help to have someone massage it out a bit to relax the muscles. A warm compress might do the same.
Siggy check.