Had them at the backs of both knees some years ago .Very painful .Prescribed Athrotec and anti inflammatory gel .Bloomin' painful .One of the signs that you may be getting osteoarthritis in the knee .They cleared up eventually .
Have a look here
http://www.bbc.co.uk/...ons/bakerscyst1.shtml
I had one a couple of years ago,not painful or anything.
Doc said if it gets bigger to go back to her,anyway it started to shrink..which she said is common and now it has gone.Hope this gives you peace of mind.