My knee got sow thing that feels like a bubble. it seems like it's from a past injury, but i didn't do anything that day. why could it of come back?

Knee sore: Is it swollen? With past injury the knee is susceptible to develop an effusion/ swelling with overuse or if no effusion knee tendinous inflammation can occur with overuse/ prior injury ( it never goes back to 100%). Motrin like products if no allergic, elevation, ice 10 minutes on 3 x a day, no strenuous activity, and kinesiotape can help. f//u w/ doc

Knee effusions: or excess fluid in/around the knee joint are usually the result of trauma (to ligaments or meniscus), arthritis, or infection and often result in swelling and soreness. If small, may not need aspiration for diagnosis or treatment, but the fluid analysis can be very helpful in determining the cause.Very common to recur without identifiable reinjury. Rest, elevation, ice, NSAIDs can be done at home