Hi and welcome to HCM.
Well, sometimes even doctors cant find the cause of your pain. This can be cause dby nothing or by various disorders. Considering your info this should be related to your reproductive system so the first thing to do should be pelvic and vaginal examination and ultrasound
. There can be ovarian or uterine pathology, std
. ALso depo shots can have side effects including abdominal pain
. I am sure that it will pass soon,ish you good health.