The cat you mentioned was probably treated with flea treatment meant for dogs instead of flea treatment for cats. That can kill them.
I know that vets can be expensive but in my opinion there is no comparison between products you get through your vet and those you buy from the pet shop - the over-the-counter ones are inferior and I've never found them to work very well at all. They are a waste of money. At least when you've got stuff from the vet you know its the right stuff and it'll do what its supposed to. Now, if my dog was lethargic and drained, I wouldn't be writing on this, I'd be at the vets NOW. If the dog had a very heavy infestation of fleas, it could be anaemic and may need an immediate blood transfusion.
Or, the flea treatment may have poisened it. I'm not trying to worry you, and I may be wrong as I haven't seen the dog, but going on what you've said, at the very least, I'd phone the vets and ask for advice.