I could make a guess, but in all that is good judgement: GTF to a doctor.
If you have a torn ligament or broken cartilege, you need to get it repaired or you could be just walking along one day then POP! It’s probably just a sprain, but don’t take the chance. Knees are a LOT easier than head and neck. It’s probably just a bad sprain that will fix itself with a few days of ice and elevation, but the piece of mind is worth it.
If you’re skeptical about getting a bad doctor, go to Dr. Paul Peartree in Rochester. He was one of the best orthopedic surgeons 10 years ago when he was taking care of me, and now he’s got 10 more years of experience. If my knee ever gives me more trouble I’ll be going back to him.
You’ll thank yourself when you’re not walking with a cane at 40 because you wouldn’t go see a doctor when you fucked up your knee in your 20’s.