What is this swelled up area behind my knees?

Could it be arthritis?It's about the size of a small egg and on one knee it's bigger than on the other knee.When I walk or stand a lot,it gets bigger and my leg feels sort of numb.When I sit and rest it goes down.
An orthapedic doctor told me a couple of years ago it was a cyst.If so what should I do,if anything,about it?
A lady friend who has expierence with older adults and health issues said this cyst could wrap around the nerves and cut off my circulation.
I would like to hear from a nurse/doctor or anyone who knows about this problem.I am 61 yrs old,female.I also have degenerative disk disease,could it be coming from a pinched nerve?
I haven't been to a doctor for this yet.
Thanks in advance.

Is exercise a good or bad thing?My knees are stiff every morning and before it rains.
Is there anything I can take to relieve the fluid without surgery?

you more than likely have a bakers cyst in the back of your knee. Since you already know that you have arthritis, thats pretty much how you get a bakers cyst. Some Doctors will actually aspirate it in the office. with no surgery.. others need to remove it through surgical intervention. You really need to see the Orthopedic Doctor again to see what he can do for you, and to see what he recommends with your age and condition. Physical Therapy may be good for you as well, or a certain brace. Good luck with that!

Probably a Baker's cyst and needs surgical removal.

It does sounds like a cyst. If it is a ganglion cyst (the one that affects nerves), they can be removed surgically; however, there is no guarantee that it wont grow back. If either are causing you pain, then you really should have it/them removed. And I doubt that your degenerative disease is causing lumps. A cyst if a sac filled with fluid. Most times, the physician can perform a needle biopsy in the office. Talk to your doctor about it. Im sure if he knows its causing you pain, he will remove it for you. Ganglion cysts do not routinely cut off circulation. They are more like little hard lumps you will feel upon palpation. If you were to accidentally hit it via an accident of some sort, you will notice that the small lump will actually "break up" into many smaller ones and scatter (under the skin), then eventually join together again. Its sorta fascinating in a way. But, go see your doctor!