One common cause of pelvic pain, painful periods, as well as painful sex and painful bowel movements is endometriosis.
Endometriosis is a condition where cells normally found in the lining of the uterus/ womb grow elsewhere in the body e.g. in the fallopian tubes, ovaries or along the pelvis. These cells, however, still behave as though they were in the womb, and break down as menstruation. This leads to the chronic pain, cramps, heavy periods, and other symptoms that characterise the condition.
The exact causes of the condition aren’t known, although there are several different theories.