Then the dosages are going to be different I'm assuming.

The vet might even be able to tell you what concentration your current meds are, and advise you how much to inject into the smaller animal. That way you wont have to buy more.

What were you giving the one that passed and how often?

Try not to beat yourself up, just double check your husbandry and try to learn as much from your mistakes as well as others to try and provent them from happening again.