Since august ive suffered lower back pain/hips and abdomen tenderness directly below my belly button (not moved). i am a thin person and the pain appears deep when touching what feels like a long solid vein/muscle. aaa exam negative. what could it be?

Seems unusual: If this pain is enough that it interferes with your usual activity or well being then it needs further evaluation. If it is unaffected by movement then we can assume its not musculoskeletal. If you are truly tender in that spot then you need urinalysis, blood work, and if still not clear an abdominal and pelvic CT scan with contrast. This can be done all at once in an ER or over time at a clinic.