Hi this is Dr.Subhadeep tripathy. thanks for the opportunity to help you.it is not very clear from your description whether it is outside or beneath your knee joint,so will guide you for both the conditions: if it is beneath the knee joint it can be due to gout or rheumatoid arthritis.in that condition you can do a total count and test for serum uric acid tests and also x-Ray of knee joint.if serum uric acid is more and total count is more than you are suffering from acute onset of the abovementioned conditions.if gout or arthritis is confirmed,consult a rheumatologist.he will prescribed madras,steroid(low dose) and allopurinol in gout. If fluid accumulation is outside the joint space it may be due to joint effusion and pre patellar bursitis.it can also be tender,tense swelling with warmth showing signs of infection like arthritis either rheumatoid or osteoarthritis or underlying bone diseases.in these conditions following tests can be done: CBC,joint fluid aspiration and biochemical,microbiological analysis,x-Ray knee joint,MRI knee joint. treatment options: weight reduction and lifestyle modification in obesity fluid aspiration from joint and injection of steroids to prevent further inflammation ice packs,NSAIDs,paracetamol,rest. consult orthopedician if the diagnosis turns out to be per patellar bursitis and follow his advise. Regards