Why does my back hurt sometimes, especially after lifting weights but y doctor says im fine?

Muscle: It is quite normal, in fact some weight lifters consider it desirable, to have pain in muscles you exercise with weights. Even without lifting you can strain your back with just a wrong movement. It is a common reason people see a dr. Left alone most back pain goes away. If it continues or worsens, despite rest, see a dr. Even then, it mostly just takes time to heal. You're 18. Don't be in a rush.