My symptoms were stomach pain (so bad I had to lie down) and constipation on day 3 of my period, and then sometimes in whatever part of my cycle I could feel a soreness in my left ovary when I went running. Before that for a while I'd had IBS on and off.
Then when I was staying with my parents for a week or so and thought I had a stomach ulcer- really sore, sharp pain, and my stomach was bloated.
Went to GP as soon as I got home. He took it very seriously in case it was endometrial cancer or ovarian cancer, CA 125 was elevated. Had scans and then keyhole surgery, endometrial cyst on on left ovary, endometriosis adhesions all over bowel, bladder. Removed the cyst and some endometrial tissue. Put back on mini pill, copper coil removed - that was 8 years ago and I've had no problems since. More recently for the last year I have been taking oestrogen as well (quite a few menopause/peri symptoms) and was worried about the endometriosis flaring up again but it has been absolutely fine and I feel so much better for the oestrogen.
I also took up yoga, changed my diet a bit- less processed food certainly, less carbs, less meat, more veg, more walking, have been back in the gym regularly doing weights and cardio since starting back on the oestrogen- pelvic floor even seems better doing high impact exercise these days, bit of running again- I have more energy, so I feel really well at 48, certainly better than I did at 38 when I was having several unpleasant symptoms.
So if it is endometriosis it can be managed but you may need surgery and will certainly need the right pill/hormones. Diet and exercise can help but you need the hormones. I am also lucky - some people spend years being misdiagnosed or trying all kinds of treatment.