ScanScan, I have a similiar problem for about 10 years. In my case, I think it is due to too much abuse from weight training when younger, and road running in latter years. Mid buttock pain which will eventually translate up the back and neck, and if ignored, will result in back spasm and eventually siatica. I've seen everyone except a surgeon. Doc, Physio, Osteopath, Physical Therapist, Quack etc. All would agree it is probably a torn sacroi-iliac muscle/ligament running over the ball-joint at the top of the bum, and not a compressed disc.
All above therapists helped in some way, but taking it easy and not overstressing your back is most important. Feel the signals and know when to ease up. If you don't, a disc WILL give way with the results you now have.
On reflection, on each occasion that I have been bed-ridden for 3-4 weeks at a time, there was a pattern where I overdid things and should have been more careful. We are not as young as we think we are.
Gentle stretching excercises work well for me, a physio should be able to advise what's best for you.
If you have siatica now, you need to rest your back, and start an excercise regime. It will come right in time if you are careful, I play golf twice a week, but cannot run. Lifting is a no-no.
There was a long thread in this forum earlier this year, a lot of good advice there.
Stay up straight and fly right!! Hope this helps.