Could be nothing, but could also be something dangerous - an aneurysm or clot in the leg.  Could also just be a cyst.

http://www.southcoasttoday.com/apps/pbcs.dll/article?AID=/20090524/LIFE/90524...

DEAR DR. DONOHUE: About four months ago, a bulge appeared behind my right knee. It didn't hurt at first, but then it started to give me a little pain. I saw a doctor. He poked around and finally said it was a cyst. He told me to put a hot, wet towel on it. I did this three times a day for the next three months. Nothing happened. It's still there, and it might be bigger. How does a cyst get there? Is there something else I can do for it?

— N.O.

That's a Baker's cyst. It's a soft bulge on the back of the knee. It's sort of like housemaid's knee, a swelling on the front of the knee, something that happens to people who are on their knees a lot — carpet layers, gardeners and people who scrub floors.

Behind the knee is a bursa. Bursas are small, flat discs that act like ball bearings. Nature placed them between tendons and bones to prevent friction as the tendon rubs across the bone. The behind-the-knee bursa is connected to the knee joint by a small tunnel. Any process that causes inflammation of the knee joint increases fluid production inside the joint. Excess fluid drains into the bursa, and it swells. The drainage tunnel has a one-way valve, so fluid can't drain out of the bursa. Housemaid's knee is the same process, but the front bursa does not connect to the knee joint.

Moist heat is a valid treatment for a Baker's cyst. It's obviously not working for you. Anti-inflammatory medicines like Advil and Aleve can be helpful. However, after four months without any improvement, you now ought to see an orthopedic surgeon.

The doctor can drain the bursa and inject cortisone into it to prevent fluid from reforming. And he or she can examine your knee to see if there is real trouble there.

You should make the appointment soon. Baker's cysts can break, and that is a painful experience.

Incidentally, other conditions in the same location look very much like Baker's cysts but are much more worrisome. An aneurysm of a leg artery or a clot in a deep leg vein has an appearance similar to that of a Baker's cyst.