Till now you seem to be asymptomatic - which doesnt sound like endometriosis to me as it is a chronic illness and would be like having much more persistent issues ongoing for year and onset much longer ago.
Also your symptoms are very unspecific as in they could be gastrointestinal related or gastroperiss, something like an infection, cancer, autoimmune etc. It could be ovarian cysts or fiborids, jt could be your stomach or bowels (ulcerative colitis, crohn's disease etc).
You wound need to try the mirena coil and If the mirena coil and NSAIDs resolve it long-term then you likely have asymptomatic or less disabling endometriosis/ perhaps adenomyosis.
The stages of endometriosis are actually about how it impact your fertility - so a higher stage more impact on fertility but not pain. So a person with stage 1 endo could be disabled by it and have severe complications but a person with stage 4 may not.
But you could also be adenomyosis where the lining of the womb grows into the muscle of it, but unlike endo is localised to the uterus only. Thus, symptoms are mainly a couple days before a period and during your period. Main endo symptoms:
• Irregular or heavy periods
• Pelvic pain
• Pelvic pain on opening bowels (dyschesia) and wider gastrointestinal symptoms (diarrhoea amd constipation)
• Pelvic pain on passing urine (dysuria) and bladder symptoms sometimes
• Referred pain to the tops of the legs or back
• Fatigue
The main symptom of endometriosis is not actually period pain because endo is not a period condition - it's a whole body inflammatory condition where the endometriosis tissue even produces its own oestrogen and the pain is felt throughout the month not just when on your period.
Can you be more specific about your symptoms and what the investigations you had, have found so far? If your pain is severe it might be an underlying condition. It could be fibroids or ovarian cysts.
Are you symptoms just on your period or elsewhere during the month? (Because if its just on your period, unlikely to be endometriosis).
What symptoms do you have during your period? And if you have symptoms not on your period - what are they? Is the pain worse on or off period?
If you had a transvaginal and/or pelvic ultrasound normally it can pick up adenomyosis. I recommend going to your GP ask for a pelvic ultrasound, a transabdominal ultrasound and a transvaginal one to see what they find.
You may have 1. Primary dysmenorrhea (heavy bleeding and painful periods and pelvic pain with no condition or cause or 2. Secondary dysmenorrhea so a condition e.g endometriosis or adenomyosis causing it. You will need to try NSAIDs and mirena/pill and Tranexamic acid first before any further scans like ultrasound and MRI. Often if the pain is significantly reduced with said treatments its more likely (not 100%) to be primary dysmenorrhea. Also very dependant on whether you only have symptoms on your period or across the month as to whether you have primary or secondary.
Then if ultrasounds pick up on anything (pelvic ultrasound or transvaginal) you may need to have an MRI to see endometriosis. However, the only way for definite diagnosis is a laparoscopy and then they will often excise the endometriosis tissue if they find it etc. But endometriosis is a chronic illness and cannot be cured, after surgery it will grow back, it cannot be excised from everywhere to leave organ functioning intact and then post-surgery adhesions often form.
I wouldn't go down the gynaecology route just yet - stick with your GP and investigate everything - bowels, liver, gallbladder, stomach etc and also ovarian cysts, fibroids etc. Once you get more definitive ideas from scans, bloods, stool tests etc - you'll be able to narrow down a speciality to go to if further secondary and specialist intervention is needed.