About three weeks agos I began developing a sharp pain on my left knee on the inside of it. It only hurt when walking, but when I was resting it was fine. It recently began hurting constantly though and my knee gave out a little bit once. Whenever I bend my knee there is a popping or clicking sound too. I want to go the doctor to get it checked out, but my dad claims its just growing pains (I'm 16) or I just sprained and it will heal in no time. What do you guys think about it, shoud it get looked at, does it sound serious, etc.

I would go to the doctors. Just in case your dad is wrong, this could be getting worse every single day.