Tightness in my left knee when using stairs?

This has been going on since late January and it's becoming more and more frequent. It used to be my knee would only feel tight (on the outer left side of my knee cap) if I went up or down the stairs, but lately I've been getting that feeling if I've walked for a while, used the stairs, running (I don't run very fast, I'm afraid my knee might give out if I do) and just bending it a few times can cause it. My knee isn't swollen, it never swelled up at all before either, I can touch my knee when it feels tight and I have no tenderness. I've asked my doctor, who's requested I get a few x-rays of my knee done (I haven't gotten them done yet), and he couldn't figure out why it might be from the way I described it (it's hard to describe). He looked at my knee and bent it a few different ways, tried moving the kneecap around, but he still had no clue. I haven't injured it in any way, I snowboard during the winter, but my right leg is first. I'm worried if I pulled something or anything like that. Like I've sai, it doesn't always hurt. Any ideas of what this could be and how might it be treated. Thanks!