Swelling behind both knees and gets stiff if standfing to long. i am a hairdresser and work out 4 times a week doing kick boxing and zumba.

Need a diagnosis: Treatment is diagnosis and not symptom driven. You need to see a physician and if appropriate get x rays to see if you have arthritis in the knees. If you do, conservative treatment which may include avoiding impact sports until your symptoms abate.

Standing: For long periods of time can reduce venous return leading to blood pooling in the lower extremities. High fitness levels will improve blood flow throughout the body but venous return is gravity dependent.