Feanor,
This sounds a lot like something that happened to me a few months ago. I assumed I'd pulled my hamstring and so did my doctor. I went through all the therapy including ultrasound to improve it, but nothing really helped.
Here's what was really going on. My hip flexors and quads were way too tight. They were pulling on my pelvis causing it to tilt forward a little. This in turn overstretched my hamstring until it got really irritated where it attaches to the pelvis (in the glute area).
Like you, I didn't experience a sudden onset of pain that I had associated with pulled hamstrings - like when sprinters fall out of a race. What I had was a aggravating, pulling ache where my leg meets my butt on my front leg. I found myself grabbing my rear a lot doing footwork (kind of embarrassing). It was the worst when I was retreating and changing direction.
I ignored this for a long time and eventually did strain my hamstring in the middle of my leg. The therapy helped that a lot, but the ache near my rear never went away.
What did work was stretching my hip flexors and quads really well. Stretching my hamstring even gently (recommended for recovering from a hamstring strain) actually irritated it more. Even now occasionally I'll feel that posterior pain, but if I stop and stretch my hip flexors it goes away.

Now that it's better, I'm careful to stretch both my hip flexors and my hamstrings to try to keep everyone happy.
My two favorite hip flexor stretches:
1. While standing, grab your heel behind you with the same hand. Do this really common quad stretch, but be sure to stand up as straight as you can. If you lean forward you miss the hip flexor part of the stretch. You can do it in front of a mirror to check yourself.
2. Start at a kneeling position with one knee up and one knee down. Stretch the leg that's down out behind you so that your knee is no longer at 90 degrees. Lean forward on your other foot until your knee is a little past your ankle, (Don't freak out, people; it's not a lunge.) Put your hands on your hips and then lean back keeping your torso as long as possible. I think of going backwards and up and making a big "C" with my back and back leg. You should feel this through your hip and groin on the side of the back leg. For more of a stretch, take your hands from your hips and raise them by your head.
If you've got what I had, you're dealing with inflammation from an irritated attachment site, so ice and anti-inflammatories will help the pain the most.
Try these stretches, but, of course, use your judgement about what's going on and what helps and what doesn't.
Best of luck,
Susan