i'm still a teenager, and i'm not sure when this first developed, but whenever i try to do situps on the ground or even when i laugh really hard, my middle/lower back just hurts. it's kind of like a burning feeling. i've mentioned it a year ago to my physical therapist whom i used to go to for my knee problems, and she shrugged it off and told me it was probably because i wasn't keeping my back straight when doing curl-ups or crunches. either way, it's starting to effect my workouts because i can barely do curlups without my back hurting. im thinking that it might be linked to my knee problem. any idea on what could be wrong with me??