The goal of this clinical trial is to learn if NSAIDs (i.e. naproxen sodium) can treat menstrual pain and prevent the development of chronic pelvic pain in menstruating adults with painful periods. The main questions it aims to answer are: * Can non-menstrual pelvic pain reduction be predicted by menstrual pain response to NSAIDs? * Will participants with the largest reductions in multi-site sensitivity following NSAID therapy have the largest reductions in non-menstrual pelvic pain? Researchers will compare naproxen sodium to a placebo (a look-alike substance that contains no drug) to see if naproxen sodium works to treat painful periods. Participants will: * Take naproxen sodium or placebo during several days of their menstrual period every month for 1 year. * Complete computer questionnaires and tests from home every 3 months. * Complete at-home urine tests to measure hormones every few days for 1-year. * Use a pin-prick to collect a small spot of blood, and use a pad or tampon to collect a sample of menstrual blood, and bring it to the research site twice over a 1-year period. * Come to the research site twice over a 1-year period to complete sensory assessments and undergo a blood draw. The major goal of the study is to develop a multivariable statistical model (see https://grants.nih.gov/grants/guide/rfa-files/RFA-NS-24-021.html ) describing the factors that effectiveness of pain medication and risk for chronic pain