The fact that you have developed this menstrual cycle disturbance recently associated with pain is suggestive of some infection in the pelvis. You should see a specialist for a proper evaluation and vaginal swabs may be taken if feasible for culture tests. Urinalysis
should also be done along with a trans-vaginal sonogram to exclude pelvic causes. If everything is fine and pregnancy
is excluded, though this is a remote possibility, breakthrough bleed due to hormone insufficiency should be suspected and treated. Hope this helps.