What Causes Popping Sensation In The Knee Post Workout?

I was doing a Barre workout this morning (a plie in horse pose or 2nd position) and the inside of my knee, just inside/under the knee cap popped. The pop was on the down/plie move. Very unusual for me...I could keep going but had to adjust - if my foot was strait on it was better. But now as the day moved on, there is pain on the outside of my knee as well. So though you can t look at my knee and see swelling, because the pain is increasing somewhat, I know there is swelling. I have taken Advil. Just wondering if I should give it a few days or go have it checked.