My knees hurt after karate and anti-inflammatories don't help. What should I do?

Hello, I am 24 years old and I often (almost every time) have pain in my knees after doing physical exercise such as karate (1x/week) and lately racquetball. The pain comes on during exercise and stays for several hours, sometimes preventing me from even standing to cook. Often, it is only after a night of rest that it passes. The pain is diffuse, I could not say which precise place of the knee hurts me, if not rather near the patella. The anti-inflammatory drugs (ibuprofen) do not have a noticeable effect. I have never had a knee injury. Would it be worthwhile to use a "support" for my knees during my activities or to try an anti-inflammatory cream or is there something to try as a prevention? Thank you.