Our dog has lumps on her hind leg. What do we do?

[Kolesar]

Our 12 year old golden retriever has been aching a bit and not wanting to go on long walks for a few weeks (long is 1 mile). She has a lump on her hind leg at what I believe is her "knee joint". It was hard a few weeks ago, and she snapped at me when I would touch it (it hurt). Now it is softer (as far as I can tell), and it actually looks like two lumps, one forward of the joint and one a bit behind. The large lump is as big as a quarter. The lumps move around with the skin when you handle them.

At first I thought she had an inflammed sprain, but now I don't know. I am suspecting water on the knee, or maybe some sort of tumor, as pure bred dogs get. She certainly does favor it, even when she is standing.

We are taking her to the vet in a few days. I would like to read up on it, but I just don't know what terminology to search under. Does anybody have any idea what these lumps may be?