Should I get my knee checked for a tear?

Today when I was doing leg exercises, I noticed something was wrong with my left knee. Whenever I stand up straight on one knee and bend my knee with my weight on it, I can easily do it with my right knee but can't really do it with my left. My left knee just buckles under the weight and I stumble over. I've never torn any ligaments (that I'm aware of) and never have I had a serious injury before (like a broken leg, etc). What could possibly be the problem?