Are you getting these when you are on a period, or when you are ovulating between period bleeds?
It certainly sounds like an ovulation pain can feel. I'd get a sudden sharp stabbing pain, enough to buckle me over sometimes, then after a few minutes may be another or just the one, but afterwards it cramps a bit and aches and I might get a bit of a bleed or just spotting but not always. Then it calms down till the a period arrives.
My sis doesn't have endo, I do, but both of us could always tell when we ovulated, which side and which month would be twins !!
If the cramps are happening when you are actually on your period, count yourself lucky the cramps ease off after 20 minutes, mine last for days, even with lots of pain killers. There is no respite once it gets going, that me written off for few days every month.
If you haven't yet had a scan, that speak to your GP to check for cysts or any other obvious swellings that might account for a sudden cramp when you mid way on a period.