Why does mittelschmerz last a whole day?

Fluid to reabsorb: When the egg releases from the ovary there can be a bit of bleeding, and the follicular fluid also is released. That fluid and blood may be irritating to the pelvic tissue for several hours as it is being reabsorbed. Some tylenol (acetaminophen) or Ibuprofen may sometimes help. See your gynecologist if the pain becomes significant.