Question for those of you with bowel endometriosis

I am not officially diagnosed, but I believe that I have endo. I have been going through the process of ruling out other issues with various doctors and have some questions about bowel endo symptoms.

Background: I have recurring pain in my lower left abdomen that seems to coincide with my cycle. It's weird because the pain subsides during my period, but once my period is over it comes back. I don't really have painful bowel movements, but starting in June/July I began to feel some discomfort in my lower left abdomen accompanied by (here comes some TMI) constipation, mucus, and narrow BMS. I have had both an ultrasound which looked perfect and a colonoscopy which also looked perfect so I'm kind of at a loss. Also, since the start of 2022, I have been having prolonged periods (from 7 days to 10+ days), migraines with aura at the beginning of my periods, and increasingly painful cramps.

For those of you with confirmed bowel endo, specifically in cases where the endo is on the sigmoid colon rather than the rectum, what were your symptoms and how did you go about getting a diagnosis? I need advice because I am TERRIFIED of getting surgery and having nothing be found, but at this point, surgery would be my next step.

Edit: My pain level with the lower left abdominal pain varies from day to day. Some days it is very mild and is just uncomfortable and some days it's sore enough that it hurts to walk, sit, etc. The pain can range from my lower left abdomen just below my belly button all the way to my thighs and butt.My period cramps have gone from about a 7/10 which can be reduced to a 5-ish with OTC meds, to more like an 8 or 9 that isn't really helped much by OTC meds. The pain has gotten bad enough to make me cry and feel nauseous, but I've never vomited from the pain.

Edit (additional question): when you have painful BMs, where are they painful?