What's wrong with my knee? ?

It pops and grinds against the bones sometimes and it hurts A LOT for a long time. It hurts more at night after all my activities and sports. Other times it is completely pain free. It starts to feel really sore after it pops around the kneecap and to the side. Of the knee. The pain also switches from left to right knee because sometimes my left knee hurts really bad and sometimes its my right knee. It has been bugging me for about a year now and i have worn an elastic brace when i am doing a physical elactivity but it still hurts. Anybody know what might be wrong with my knee(s)???? And i wouldve gone to a doctor a lot sooner but my mom doesnt think its a big deal so she wont take me.