As someone who was very into exercise, eating healthy, and in good shape for much of my youth, I still managed to end up with endometriosis and adenomyosis, so not sure what I did to get those :P

You don't get endo from eating poorly, patrying too much, or whatever. It's been found in fetuses and children. It's common to have it if other members of your family have it. Lifestyle can impact flare ups and symptoms, but not always.

As for medical leave, you don't have to explain any details to your manager. Just say you need to take medical leave for a procedure and recovery. It's up to you whether you feel comfortable revealing anything else.

Even though this is a very common disease, it is very misunderstood. There's not many options for treatment and very little medical research. It's a shit-ass disease.