Eeks! That is a very large area affected...the top where it is more pale could potentially be a burn, however it could also be an infection of either fungal or bacterial variety. What may have started as a burn may have become secondarily affected. Minor burn, and for the most part even the more significant burns, should not cause a change in behavior. The fact that she's lethargic definitely worries me. If there is a fungal or bacterial infection now then it could spread to the rest of the body and cause a life threatening infection. I don't say this to scare you, but I do want to stress the importance of seeking treatment as soon as possible knowing what the consequences of delay may be.
The black stripe also worries me. It could a problem on its own, like a more severe burn site that has caused the skin to die already, but based on the perfectly rectangular pattern I'm more worried that this is due to damage above where the stripe begins. Color change in chameleons is controlled primarily by two things: hormones (stress, courtship, relaxation, etc) and the nerves. If the nerves going to an area of the body are damaged then that can cause a local color change from the area of the damaged nerves. With a burn usually the area is circular or somewhat irregular. I've never seen one with perfectly straight margins like that because heat radiates. So if the burn occurred on the area just above the black stripe, it may have caused enough damage to penetrate deeper than the skin and actually damage the nerves that drop down from the spine and end near the bottom of the body. They fall in lines perpendicular to the back, kind of like ribs, only straighter. So if it was damaged above it would affect the entire region of the nerves below.
Now if that's the case, which I suspect but can't confirm since I'm just looking at pictures, then the damage is pretty severe and I'd definitely be worried about a growing infection. Your girl needs to see a knowledgable reptile vet (not just any vet) for some hands-on evaluation to look for other clues, diagnostics and most likely treatment. For tomorrow you can put some antibiotic ointment on it (never cortisone) but she should really see someone on Monday.
The problem with the room being cooler is that your chameleon will sit under the heat lamp for longer and get closer to it than normal trying to bring its body heat up since the rest of the environment is cooler. So even if the basking temperature seems lower than you'd expect to cause a burn, the prolonged more intense exposure can cause burns anyway. And just like people who get sunburns, the damage isn't apparent until it's too late unfortunately.