I've put some time into reading your posts. Just saying that up front.
While I despise Petco selling animals, I can tell you the odds are it had nothing to do with them based on the time you had the animal and the that beautiful picture of him you posted.
I am not saying that to pick on you, quite the opposite. The biggest hurdle for any chameleon keeper is to learn what works, what doesn't, and also that sometimes despite everything being done right, **** happens with our wonderful animals, just like with people. While I would not recommend buying any reptile from Petco, you had a good one it looked like.
When chameleons are sick, they suck at hiding it. However like others have already posted, I would lean towards something fast, traumatic, that caused internal injuries. A bad fall for example. Poison? Anybody use raid or something like that nearby in the last day or two?
While these animals have a rep for crashing fast I learned over time that it was not so much as they crash fast but that people didn't know the signs of a health problem. Once those signs are so obvious anybody who has not even owned a chameleon can tell it has a problem there is usually no time left to save it. Hence the rep.
Very sorry for your loss. Don't give up. Instead figure it out, learn from it if you can, then continue with the hobby.