Endo Attack

I was on the treadmill (thankfully at home and not in a public gym) and it hit hard and out of the blue. It doubled me over and sent me to my knees (also thankfully I reached out and pulled the "stop key" as I went down). Panting and sweaty, I lay there, curled up in a ball on the floor, clutching my stomach. Did I move wrong? What triggered it?

This is why I live in fear. This is why I am so jumpy and nervous. You never know when it will hit. But what can you do?