Why could my knee be hurting?

My knee started hurting like 2 hours ago. This isn't the first time this pain has suddenly appeared. On a scale of 1-10, it hurts about a 4. I'm a teenager, so my stepmom thinks its just growing pains. But I know that this isn't growing pains. I play sports and get injured, if this matters. Please help. Thanks. :) Oh, and the pain is right below my kneecap.