I have discomfort in my knees, it comes and goes. usually after i apply heating pad on the knee, it feel better the next day. what can be it?

See depends: Arthritis or tendinitis are two possible causes. Have the knees evaluated if you are concerned.

Can't say for sure: there can be many reasons- joint, cartilage but usually not both knees or soft tissue like Tendonitis or bursitis. If there is joint swelling suggests arthritis and be swelling from bursitis and tendinitis also. You need to see your physician to help clarify what the problem is.