Having had three knee surgeries on my right knee and am headed for a knee replacement at some future (hopefully at least 20 years from now), knee problems suck. Based on what you are saying, I am going to place my bets on a torn meniscus. That is what caused my third surgery. Unlike the previous two, I never did anything that I was aware of to cause it. One day I was starting to have pain again in the knee, much more then the typical overdoing it I would get in the past. I would have good days and bad ones but almost any day I did too much walking or especially climbing stairs, it would always be worst and swollen the most.
After having it repaired, most of the problems went away but honestly, once you start having knee problems, they will pelage you forever. A number of years after my last surgery (in 2000) I started having problems with pain and swelling again. Luckily I got into a very good rheumatologist who got me started (after the first attempt with another product) on a SynVisc injection treatment, which has been doing wonders for the last 3+ years. I have been able to be more active than I had been in years prior. I still have good days and bad ones. I alway know when I have overdone it, as my right knee becomes very warm to the touch.
If you are still having problems by tomorrow, I think a doctor visit will be in order. Hard part is getting the right specialist that can properly diagnose what the problem is. General practitioners don't typically have the ability to diagnose it and should send you to a specialist.
Good luck bud!