23 year old female. Chronic spinal creptis. Cracks/pops with every body movement/adjustment. Painful to straighten back or sit long. Possibilities?

Inflammation. You may have some soft tissue inflammation or deconditioning causing your symptoms. Unlikely to be any significant pathology at your age. It would be reasonable to treat with physical therapy and NSAIDs, and if you don't improve after that, then an MRI would be helpful to rule out any significant pathology.