Just another chiming in to say I don't think it's odd that a baby is allergic to an antib. though I have no idea how common it is. We discovered my ds was allergic to 'cillins (ampicillin) at 5 days old when he broke out in a rash from amp that he was given in the NICU for possible infection. It wasn't that surprising since my mother, sister and I are all allergic to cillins!
This definitely does NOT mean that she will be allergic to all antibiotics, there are plenty others to choose from. DS is now 2, and he's had to have antibiotics for various other things, and he has done fine with other families. There are some families that tend to have a crossover reaction, that is, for example, about 2-10% (I've heard varying figures) of those allergic to cillins are also allergic to keflex (sp?). However, ds just had to have keflex recently and he didn't have any problems. For me personally, it is sometimes limiting in that cillins are the "first line" of antib. for certain things, and so they'll have to give me the "next best", but that's only rarely been an issue.