What Causes Loud Pop Accompanied By Extreme Pain In Knees?

I was about to get into the tub this morning, and heard a loud pop accompanied by extreme pain in my right knee! My left one already has arthritis, so I immediately nixed the bath idea and took a seat on the toilet to regain my composure. After several minutes I tried to stand...nothing doing! Too much pain and the left knee isn t strong enough to support me. I am able to crawl, very carefully however, without too much pain (no laughing! okay, you can laugh! I am!). Not quite sure what happened here, but has sure taken me down in a hurry! Does this sound like more arthritis? I have experienced a little more pain in the knee for a couple of days preceding this event. Something about this feels different.