Mittelschmerz is pain associated with ovulation, and is usually not due to any disease or serious condition. Let me briefly review what happens during a woman's menstrual cycle. Day 1 of the cycle is the first day of menstruation, typically lasting three to six days. Menstruation is nature's way of getting rid of the lining of the uterus since pregnancy didn't take place during that cycle, and preparing the uterus for the next cycle. As soon as menstruation stops, the lining starts to rebuild itself, and at about day 14 the woman will usually ovulate.
Ovulation involves the release of an egg from one or the other ovary.
The egg is released through a small rupture or tear in the surface of the ovary. In most women this rupture is painless, but in some women, for reasons that I don't believe are known, the rupture causes pain. It may be that women with this condition have a little bleeding at the site of the rupture, and that this blood, which is irritating to the lining of the pelvis, causes the pain. In any case, the typical history is of a pain which may last for several days and that comes regularly between the menstrual periods. One of the easiest ways to treat this if a woman is not interested in becoming pregnant is to take birth control pills which will stop her from ovulating, and therefore should totally prevent the pain.