Wow! I had a similar issue back in 2016. Pain in my left butt cheek at the very base where it meets the top of leg. It also radiated into middle of butt.
. I COUND NOT SIT in a chair at all. I leaned my bottom against a stool. Went to doctor and she told me to wear compression underwear and also sent me to PT. I didnt get any MRIs for that but had a fantastic PT that gave me lots of exercises to strengthen. Seems that my left is weaker than right. I used tens unit on it, ice and heat.
It is not gone but its under control. I'm just very vigilant and try not to over do my sports....I cycle, swim, hike and walk.....I ran at one time but gave it up for biking.
I read an aticle about high hamstring Tendonopthy in was a case study. If I find the link I'll get it to you.
Another thing that helped was deep tissue massage....not the relaxation....but find a massage therapist that isnt afrain to get in there. Ann bath soak in Epsom salts
Good luck & Good Health