Sorry to hear about your cham.
R.I.P little one.
It could of had other underlying probs hun, and when buying really young babies it can be a bit hit and miss as you dont know what the pet shop where giving it to eat ie foods that where to big ect and if the parents of the little one had probs that could of been passed down.
Try not to blame yourself hun. I give my chams a good spray once a day just over the leaves not over the cham itself.
To be honest hun with out a vet determining cause of death which could be quite expensive you wont know the cause.
If you are going to get another one although babies are cute try go for something a little more established I have found this best when buying reps as I have lost a few babie reps for reasons unbeknown.
Sending you hugs hun
hope you ok. x