If you have not had your period for a month and have pain in your belly, if you have a history of sexual intercourse but have not used contraception, you should rule out any pregnancy-related diseases, such as pre-eclampsia or ectopic pregnancy. If the test result is negative, you can take oral progesterone to promote menstruation. The pain in the belly when you don’t have a period is probably because you are about to have a period and the pelvic cavity is congested, so it will cause a vague pain in the belly. However, if the pain is obvious, it may be caused by pelvic inflammatory disease, endometriosis or pelvic tumor, and if necessary, ultrasound examination and other laboratory tests are needed to confirm the diagnosis and then give treatment.