Endometriosis, fibroids etc do not just 'clear up' fibroids require surgery to remove them, and endometriosis is a chronic condition.
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 (different to endometriosis).
I recommend asking the GP to try the pill and/ or mirena coil can be amazing with heavy bleeding and pain. You might try different pills first. The mirena will may the uterus lining thinner, less bleeding and less cramps). Sometimes the progesterone only pill (mini-pill) can be better for some.
Also ask your GP for mefenamic acid and/or naproxen (NSAIDs which help a lot) and tranexamic acid (helps reduce heavy bleeding) - you need to start taking it days before your period starts so that it can work at best capacity. The same with ibuprofen and/or paracetamol. Start taking it 3/4 days before and it will be much better
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 diagnostic 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.
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. Often endometriosis on the ovaries can form cysts containing old blood called endometriomas (also known as chocolate cysts) which can be very painful. Another popular cyst is haemorrhagic ovarian cysts with endometriosis.
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
Crucially you can still have endo even if they don't see it on the scan, the only clear way for diagnosis is a diagnostic laparoscopy and then mangagement can look like pain medication depending on how severe your pain and symptoms are (can be opioids which are mainly given for temporary use for a few days at a time, NSAIDs) and contraception and hormonal treatments (gonadotrophin releasing hormones).
The links below have much more detailed and useful information.
https://www.leedsth.nhs.uk/patients/resources/endometriosis-2/
https://www.nhs.uk/conditions/adenomyosis/
Personally, I have endometriosis, adenomyosis and other unrelated conditions, I am disabled by it. Cannot walk or stand lots (across the whole month with or without a period, and use a walking stick/rollator). Drugs to stop my period do nothing as my period whilst painful is the least of my worries (even without it every day is horrible so it doesn't matter personally to me). For others it's life changing. Again mirena for years nothing (horrible initially and worsened bleeding - this happens to many people and then settles but has gotten worse over time with me. I live on high dose opiods (I wouldn't if I didn't have to but it means I'm not completely bed bound). Again I've maxed out NSAID doses and have chronic cycles of constipation and diarrhoea (going through both gastrology and colorectal surgery) and severe bladder issues including incontinence and an overactive bladder (I see urology as well), persistent thrush etc and vaginal cramps all the time which are incredibly painful.