Hi Helen,
Piriformis syndrome is truly a painful condition , especially for such a long time! The bout that I had (see my post on this thread) lasted for some time and it was really excruciating to sit!
Medical doctors may not be as expertised at treating this condition and may simply try to mask the pain with painkillers and anti-inflammatories.
What turned it around for me was a very competent chiropractor.
I hope you have an opportunity to have a consultation with a chiropractor who is repudable and knowledgable. S/he may be able to rule out other possibilities and/or contributing factors as well. The condition can be reversed with proper care.
Good luck to you!
Hope4thebest