If it's any consoltion, ANY ONE would be in some pain standing on a concrete floor working long hours.
Your knee is your Achilles heel, so to speak, lol..!! Everyone has some part of their body that is considered their Achilles heel so simply try to work less and rest more. You've said that your overtime is mandated, nice to have extra cash for Christmas, but I'm wondering if you could ask your employer to ease up a bit in your case.
Yes, your upcoming vacation will help you get some much needed rest but in the interim, it wouldn't hurt to ask your employer if they could possibly ease up on your extra hours.
A bit of swelling is "normal", in my opinion, when you're overdoing it, but honestly, you are not taking proper care. Listen to your body - you're knee is telling you, ease up a bit, girl..!! Take it as a warning sign, as you should.
Before my revision, I was working part time in a book store for fun as I retired quite early from my banking profession, and during the peak season for retail, November and December, I was working close to 60 hours a week. During one shift, early December, l remember just breaking down and crying due to the pain in my knee, I could barely walk any longer, that's how stubborn I was. It was a poured concrete floor covered by linoleum and my Fitbit registered over 12,000 steps per shift. The young bloods were complaining of pain in their bodies and being over tired...I told them to be real, look at me, l'm working with a fake knee..!!
After my employer witnessed this incident, they realized they'd pushed me too hard. Sad to say it was only then that they agreed to reduce my hours. I hope you and your employer have the wisdom to not let it come to my breaking point.