What to do or what's wrong when there's alot of fluid on the knee and a little painful?

Aspiration: It depends on the cause of accumulation and amount and associated conditions. But if it is significant and painful and you are not taking any Blood thinner, at least for the symptomatic relief the fluid can be aspirated by your doctor. Continue takin Ibuprofen unless it is acute and Traumatic, then you need immediate attention to make sure there is no Blood. Apply Ice too. Rest.

Knee Effusion: If you have fluid around or on your knee, you need to be evaluated to see if it is an injury, infection or something else. Go to an ER or Urgent Care for evaluation.