I’m not sure what she is doing when she tightens her body. I wonder if it is part of her self-soothing especially if she is doing it when she is bored or anxious?
It is good to rule out medical origins with the urge to pee before thinking it is sensorimotor OCD. Once medical has been ruled out you can take the approach of helping her handle the discomfort without getting reassurance or repeatedly peeing. Is she having accidents? Kids with PANDAS/PANS often have sudden urinary incontinence where they wet their pants.
Here are some resources for sensorimotor OCD: