Menstrual cramps are from contractions of your uterus forcing the blood out of your uterus. The lining of our uterus releases prostaglandins. This increases the contractions and can cause nausea and lightheadedness. Two weeks before your period you are ovulating and your body releases MORE prostaglandins during this time which is why a lot of us feel just plain crappy. Some women produce higher levels of prostaglandins which make cramps worse for them causing the pain can be accompanied by nausea or vomiting, diarrhoea, constipation or feeling faint plus headaches.

I bet your anus is hurting from the contraction. Ibuprofen, naproxen and mefenamic acid are best used to relieve pain and can also decrease the amount of bleeding. They work better if you take them from the time when either pain or bleeding starts or the day before a period is due. They help by decreasing the levels of prostaglandins. I've had my vagina hurt but I can not remember my anus every hurting. The part that is in between the anus and the vagina use to get sore.