My doctor has diagnosed I have a condition which he calls "mittelschmerz". But he hasn't explained what this illness is. Can you help?

The word "mittelschmerz" is just German for "middle pain" - in other words, pain occurring in the middle of the month, about halfway between periods.

So it's a symptom not a diagnosis. Therefore, I think you should ask your doc to tell you why he thinks you're getting this pain.