hello there.. I feel that your rash occurs due irritation caused by the pad and moisture that is there due to its application. Inorder to avoid these you should maintain adequate perineal hygiene. Avoid usage of irritants to clean the perineum like soap and savlon. Properly wash the perineal area with water after using the toilet. keep the area dry to avoid fungal infections.Frequent change of pads is very important (never apply them for more than occurs) as prolonged application of pad leads to rash and in extreme cases to a condition called staphycoccal toxic shock syndrome which presents with fever, vomitting, diarrhea etc. You havent mentioned if there is any foul smelling discharge and itching in the vaginal area as these occur in genital infections and can cause rash in perineum. So its important you visit a gynecologist and get yourself internally examined for the same as well as to rule out STD s. You have mentioned that you develop these rashes in menses but genital infections are more common and flare up during menses if present. Also urinary tract infections can cause rashes in the perineum so get a urine microscopy and culture too. You can use antifungal and antibacterial ointments for the rash only after you get the naked eye examination of the rash by your doctor.