It's unlikely your general doc can diagnose a knee, but it wouldn't hurt to ask. An MRI would tell you more than an X-ray, as it would capture a better picture of the softer tissues such as torn cartilage. As for your PT, yeah, ask, you'll probably end up there anyway although if it's torn cartilage you might need surgery depending on which cartilage and where it's torn. I once had a knee diagnosed as a sprain, but it wasn't. It was then diagnosed as a meniscus tear, but it apparently wasn't that either, though since I didn't get surgery I'll never know for sure. What I'm saying is, I've had a lot of injuries, but very few that were easily diagnosed correctly or treated. It's a jungle out there. Did anyone tell you to ice it and elevate it, and if it was a sprain, well, when I got my wrong diagnosis, the orthopedist sent me right to PT.