The back of my knees don't hurt but they feel really weak and sometimes they pop when I'm walking, the pain came gradually aftrr walking with weights?

Knee weak popping. May be injury to meniscus cushion between the femur and tibia. or knee cap could be mal-aligned or may be strain of the ligaments. Do not wear weights , they should only be used short term for toning and have to start slowly w/1 lbs and work up. Since you are 17 and l growing do it under guidance of PT. Rest leg ,elevate, ice 10 min 3 x/day try motrin 200 mg 1-2 x a day f/u w/doc/ortho.