Jake,
I am so sorry! Anytime one loses a pet or one of our chameleons, it is a very upsetting and sad experience.
They best way to determine her death is to take her to the vet for a necropsy to be done. Many veterinarians appreciate this and do not charge because there is still so much we do not know about Old World Chameleons and much research is still needed. My vet has continually asked me, that if I ever lose a chameleon to please let him research the chameleon for more scientific knowledge.
Aside from the normal illnesses we already know that can cause death in chameleons (i.e. MBD, parasites, egg binding, etc.), they can also die from causes that us humans experience as well. She may have had heart difficulties, tumors, and other internal problems. Just as our other pets like dogs and cats, chameleons can get illnesses and diseases too that have nothing to do with the care we give them. The only thing we can do is help provide more knowledge and research to the veterinarian community, as well as to herpetology researchers.
So again, I'm sorry for your loss and remember, it is definitely possible it has nothing to do with your care... She may have had other difficulties that caused her death. And in those cases, there is nothing that there could have been done to save her.