Part of the reason she might get sick more often is that she's no longer getting the antibodies from your bm. You can't replace that with anything, unfortunately. We're bracing ourselves for our DD's first winter without bfing and I'm expecting the worst!

A balanced diet is absolutely fine for her nutritionally and a multivitamin can't hurt if you feel she needs a boost. But there's no need to increase her milk intake if she's getting adequate calcium already. It won't stop her getting sick. Unfortunately at her age they're often playing with other small children (AKA germ carriers!) and toys that they've touched - even bm wouldn't have been able to help her completely!