I got a swollen knee, feels kind of tight and is sore below the kneecap. what is it?

Hard to say: Difficult to say without an exam but without a trauma and given your age, it's likely patelofemoral syndrome. Try ice, over the counter Ibuprofen and decrease any aggravating activity. If pain continues past 2 more weeks see your doc. Oh, and high heels are another culprit for this in women. Flats only for now. Don't be a victim to fashion.

Arthritis: By definition is inflammation in the joint: swelling, pain, feels tight and stiff, maybe red , warm. Need to see a rheumatologist: there are 100 different kinds of arthritis and related disorders.