My husband did construction and would sometimes get these in diff. places and it would fester and when he squeezed it out would finally come a sliver of something. wood, metal shavings from the balusters in hand rails. He also put heat pack to drw it out at nite.
first your age is not mentioned in your post, and if your are diabetic or not, also, if you have had trauma before or not,did you scratched the skin at this area before or not also if this is the first time or recurrent in other sites of your body .
second if you can move your knee,and stand with no pain ,good sign ,means that the infection is superficial in the skin, but if you have pain in knee movement or standing means it`s a deep infection may in the bone ,so you have to see a doctor quickly and do boold tests and pus culture also if needed X-ray to affected knee .
my post may help
Pus means there is an infection, and at this point I would see your doctor. You may have something embedded in your knee that you know nothing about, and until that is removed it will stay infected. A friend of mine woke up with extreme pain on one side of his stomach and when he went to the hospital they found a piece of metal embedded in his stomach. He was in construction years prior to this and still doesn't recall any incident where this could have happened. It would be okay to open it up so it can drain because that pus need to come out, but it's probable going to to take an xray to see what is going on. Having said that if it was me, I'd open it up and sit in a tub of hot water so it would drain better and then see if I could see anything in it. But if something is embedded and you could pull it out you could leave some behind or injure the surrounding area, so I would see your doctor. I'm betting there's something foreign in there. Good luck and take care.