Hi Hwakeunhan,
The doctors at Kaiser said I had piriformis syndrome before my current doctor put me through tests and an MRI (which only showed a slight bulge). My chiro (primary physician) told me that if the piriformis muscle gets inflamed or damaged, it lays directly above the sciatic nerve which runs up and down while the piriformis runs length-ways across the top of the butt (so if you were to picture it...it would look like a 'T'...where the top part of the 'T' is the piriformis muscle and the trunk of the 'T' is the sciatic nerve.
Your doctor is doing the right things for this syndrome (if that IS what you have). I'm sure he gave you the 'twist' test. That's where you turn your hips to the right and then to the left (instead of top of trunk twisting...your hips are twisting). If there is pain caused by this action...it is 'definitely' the piriformis, if not....you need to search further. Have you had an MRI? It is the first test doctors will "normally" give you when they need to see what is going on.
Another thought.....water therapy. Ask you doctor if that is for you. Just make sure you have a good water therapist before you jump into the water (so don't just get a YMCA membership....go through a therapist). You also need to make sure your ab muscles are good so you can get in and out of the pool....but your therapist will know to make sure you are OK in that arena.
I hope this helps. I just hope you are staying off your rump. No driving. No sitting (for longer than you can stand). No bending. No vacuuming (one of the WORST for piriformis/sciatic pain)....same with laundry. If you are on rest (per your doctor)...please REST. Do NOT take that time to do all those errands you just haven't had time for. This time off should be just that....your muscle and sciatic nerve need relief. So slow down....and rest. It takes double the time for back injuries to heal (at least) because (1) there is no blood supply going to our disks and (2) we are working with our central nervous system....we all must be careful there.
Well, I'm not sure if I answered anything here or not for you (hehe)-sorry.

But I sure hope you get some pain-free moments!
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successtory
Oct 2000: Repetitive Stress Injury-Inverted Hernia
Feb 2001: MRI. Shows only slight bulge at L4-L5
Dec 2001: Discogram/CT scan shows Inverted Hernia at L5-S1. L4-L5 & L5-S1 ruptured in all 4 quadrants. Unable to walk.
Feb 2002: IDET, Nucleoplasty, Intra-Discal Injections
Sept 2002: Rated in the top 10% for successful patients. Retraining for new career.