Hi, I have a pimple kind of thing on my knee from 7/8 years now, every time it?s little bumpy I pop it and some watery substance and skin/ white heads kind of thing comes out, sometimes not 1 2/3 at a stretch if I keep popping comes out then bleeds and I leave it, and it heals on it own. I showed this once to a doctor and he said it needs to be extracted. What is it? I have got 1 big pimple on my back which I popped and it healed and cured but got a hole there. What do I do for these?