We had the same problem with the Hartz flea shampoo.  This was over ten years ago, so it is unbelievable that the product is still on the market.  Luckily our cat recovered, but he also suffered seizure-like symptoms....he twitched, lost muscle control, and was not able to walk properly for a few days.  I was absolutely livid that Hartz could have this product on the shelf and remember calling and writing them to complain.  But, then we moved, fleas were no longer a problem, and---admittedly---I lost steam and forgot about it.

I am so sorry for the loss of Lion.  If you end up pursuing this with Hartz, I would be glad to corroborate your account.