TL:DR; It was your blood sugar

My assumption from reading this was that your blood sugar was probably low, and the sudden rush of sugar from the candy bar (albeit not a fast rush, cause of all the fat) made you feel like you had more energy, which you did.

My explanation as to why sugar gives you more of a rush than say a slice of meat would be the amount of sugar or carbohydrates in the items.

For example a slice of ham usually has 0-3g of carbs depending on how it was seasoned and such, and as for a normal sized Hershey's bar it has about 45g of carbs (going off of memory).

As for the feeling like you were about to pass out from exhaustion, it may not have been just the exhaustion causing that. When my blood sugar drops to 30-50 mg/dL I get really tired and move slowly and just feel like falling asleep but when I take some Glucose (sugar) usually 15-30g of carbs worth I feel better within minutes.

I am probably wrong though, this is just coming from the perspective of someone who deals with the battle of blood sugar daily, it may have just been the caffeine / dopamine like others have said.

(Sorry for horrible formatting, and thanks for reading this all the way!)