M.C.
I would take her in to see her pediatrician. It's always hard to say what kind of rash it may be without seeing it. If it is hives, you should still see some red around the bumps, and I would assume that your baby would show some itching discomfort (rubbing, etc). But that's not always the case. I know when I started my son on rice cereal, he broke out in a rash too - all around his mouth and neck. Part of it was just his skin coming into contact with something new, and it cleared up in a couple of days - he was fine after that. It might be a kind of moisture rash as well. But if it is really all over her neck, shows no signs of going away, and is obviously uncomfortable for your baby, you should take her to see her pediatrician.