left knee problem, need advice

first of all, I never had any problems with my knees before, but last week after finishing my ab work when I got up from the ground (all my weight on the left leg) I heard consecutive cracks coming from the knee, even my buddy heard them.. It was somewhat painful, but I was able to finish my workout (including cardio on rowing machine) the next day we went on the beach, played some volleyball and football... I felt my knee kind of popping out twice with slight pain each time but went away pretty quick. Now today I was sitting on my chair with my left leg under my right knee (don't know the english name for that position) and when I wanted to get up, my left knee started to hurt really bad each time I moved my left leg abit and I had to wait a few minutes to even be able to move ...

what should I do? wait until the pains goes away? I don't want to go to the doc and be told to rest??

thanks