Mittelschmerz before AF?!

AF is due tomorrow. I usually have mild AF cramps the day before, but today I am having what feels exactly like Mittelschmerz that I get only during ovulation. Never experienced it at this time before. Has anyone else experienced this before, and if so, do you have an idea what causes it? I know my body really well. These are not AF cramps. It's only on one side and is constant, twinging, pinching pain, whereas with AF cramps it's dull aches coming in waves.