Katniss gets burned because the Capitol saw that she was going on the wrong side, so they hurl fireballs the size of balloons at her. She gets burned, and shrieks of the pain that develops her burns. She also concluded that she hates burns, even a small one when she tried taking a hot pan out of the oven. What we can conclude from the story is that the Capitol is trying to kill her. In the movie, it shows her burn, red, and sizzling hot from the fireball that hurled itself at Katniss. Katniss shows weakness and at least she put ointment on her burn after it healed. Haymitch gave her a parachute with ointment to apply her burns. But that was after the incident with the tracker jacker attack after Glimmer, the girl from district 1, died of many stings, with lumps the size of a plum. Whatever reason why the Capitol would make such a wrong thing.
Besides, a fireball? Are they that dumb? Well, to do that, you'd have to have a machine hiding somewhere in the rocks, or something like that.