What Causes Severe Pain In Knee?

Hi, my knee has been feeling loose and slightly hurting while I have been running for the past week. Today I had a three hour soccer practice, where all we did was do long passes and shooting. My knee hurt for most of the practice and is now hurting while I walk/put weight on it and infrequently hurts by itself (lightly throbbing). Do I need to go see a doctor, or can i just keep icing it and play tomorrow.