i think it looks like baker's cyst (consult a physician)
A Baker's cyst is a pocket of fluid that forms a lump behind the knee. It is also called a popliteal cyst. See a picture of a Baker's cyst.
A Baker's cyst is caused when excess joint fluid is pushed into one of the small sacs of tissue behind the knee. When this sac fills with fluid and bulges out, it is called a cyst. The excess fluid is usually caused by conditions such as rheumatoid arthritis or osteoarthritis that irritate the knee. It may also be caused by an injury.
Often a Baker's cyst causes no pain. When symptoms occur, they may include:
Tightness or stiffness behind the knee.
Swelling behind the knee that may get worse when you stand.
Slight pain behind the knee and into the upper calf. You are most likely to feel this when you bend your knee or straighten it all the way.
Sometimes the pocket of fluid behind the knee can tear open and drain into the tissues of the lower leg. This can cause swelling and redness in that part of the leg.
Your doctor will examine your knee and ask you questions about your past health and when the pain and swelling started. Your doctor may order tests, such as an MRI, to see a picture of the inside of your knee.
A Baker's cyst may go away on its own.
If arthritis or another problem is causing the Baker's cyst, your doctor may treat that problem. This usually makes the pain and swelling of a Baker's cyst go away.
If a cyst does not go away, or if it is causing a lot of pain, your doctor may drain the fluid with a needle. You also may be given a shot of steroid medicine to reduce swelling. You may need to use a cane or crutch and wrap your knee in an elastic bandage. In rare cases, a Baker's cyst is removed by surgery.
There are things you can do at home to help you feel better.
Rest your knee as much as you can.
Take over-the-counter medicines to reduce pain and swelling. These include ibuprofen (Advil, Motrin) and naproxen (Aleve).
Use a cane, crutch, walker, or another device if you need help to get around. These can help rest your knee.
If you wear an elastic bandage around your knee, make sure it is snug but not so tight that your leg is numb, tingles, or swells below the bandage. Loosen the bandage if it is too tight.
Follow your doctor's instructions about how much weight you can put on your knee.
Stay at a healthy weight. Being overweight puts extra strain on your knee.
Super thanks a lot doc...that was very satisfying.