You're wise to remember that a bunch of unknown amateurs on the internet are no match for professionals that you can actually see. I'm one of those nameless and faceless amateurs. I've studied (because I've had) TONS of problems in the foot and ankle. I know almost nothing about pelvic stress fractures, although I have had Piriformis issues.
That being said - I have a keyboard and I'll offer my totall non-professional thought: :-) It doesn't sound like stress fracture to me. The "pain down the leg" thing sure sounds more like the sciatic nerve is angry for some reason. I just googled pelvic stress fracture symptoms and one was pain while sitting and resting. You said you don't have pain when resting. I seem to recall that people who had pelvic sfxs tended to describe it as GROIN pain, not butt pain.
If you're wigging out because all those web pages and forums have you scared it's a sfx - trust me - I've been there. In virtually every case I've self-diagnosed an injury far worse than what was actually found.
Have you tried the tennis ball test on the Piriformis? Roll around on it on the painful side and if you find a REALLY tender spot that would seem to indicate Piriformis. (And don't forget if you DON'T find a sensitive spot the sciatic nerve could be getting tweaked up at the spine.)
Good luck.
Jim