Oh MummaSue,
Please don't stress too much. My little man (13 months) has had 7 ear infections and 1 bout of tonsilitis, and hasn't touched a drop of formula. I think some kids are just sickly kids, iykwim. There is nothing you can do to prevent it. Your bmilk does an awesome job, i try to think of it like - imagine how many he could have possibly had, had we decided to go down the formula path
Your milk supply will be fine, once your DD gets her appetite back she'll be gobbling up her milk, and your body will know what to do