There's a bubble type thing under my skin. Any guesses?!

There's a bubble type thing under my skin. Any guesses?

So tonight I happened to rub my hand over my knee, when I felt a bump. At first I thought it was a mosquito bite, but on closer examination, it feels like there is a bubble of something underneath the skin. It does not hurt or anything,and I can move it under the skin a few centemeters, but it will go back to basically the original spot when I stop. I have not done anything to my knee to injure it that I'm aware of. Any idea what it is, and where it might have come from? It's not very big, like I said I first thought it was a mosquito bite.

It's a baker's cyst, synovial fluid in the lining of the kneecap. They usually go away on their own. Some people get something similar on top of their wrists. In the olden days, if someone had one they would smack it with a flat board to "pop" it. I wouldn't suggest that.
Sometimes they do get as big as a grape or bigger. My Father in law has them from working on his knees (roofing) too many years. Excessive stooping, bending, or squatting can cause them too. Sometimes they hurt, most times they don't. They aren't serious.