Piriformis links up with sciatica. Serious sciatica will stop you walking 4 hundred yards, sitting still for 10 minutes, driving more than 100 miles or even sleeping without pain.
I'd say don't run and go see a physio... I may be completely wrong, but that sounds familiar (if you are 28 it may be different to being 58).
PS I now send the physio a Christmas card.