Have Knee Grinding, Clicking, Pain When Sitting, Soreness After Climbing Stairs. Worried About Arthritis

Hi, I am a 24 year old girl. the knee grinding, clicking, pain when sitting when i sat for hours, soreness after climbing up stair or lifting up heavy objects are just bothering me a lot. The pain or soreness usually occur below my knee cap (the place where doc hits on it with a hammer to see if it reflexes) I went to the doctor last year and he just moved my knees around and then told me to take glucosamine and never explained what causes the knee pain etc. I feel so disappointed. I am worrying if i have arthritis or something worse. this discomfort pain only discovered 2 yrs ago. During the two years time, I was walking in a moderate speed, taking 45 minutes everyday to work and another 45 minutes back. I do not know if that causes the knee pain. Please help! :(