What would b a severe pain that sometimes stays and can come n go in right lower pelvis. my doctor can't find anything wrong?

SIJ: Pain on one side in pelvic region of 33 year old female, & which comes & goes, is characteristic of looseness (subluxation) of sacroiliac joint (sij), largest joint (17 sq. Cm) in axial spine. Abnormal hypermobility of sij commonly evokes same-sided sciatica. When joint properly aligned pain ceases. In females mechanical injury, childbirth, & joint hypermobility syndrome potentiate this disorder.

Pinched Nerve/Muscle: Most times back pain that is sudden onset will resolve. He should use anti-inflammatory meds like ibuprofen, aleve around the clock and even tylenol (acetaminophen) to help. If this does not resolve he should see his doctor for a muscle relaxant as well. If it still does not resolve, i would recommend him to see a spine specialist for a full evaluation and testing. He may need to get an MRI to see inside.