Why is it that my baby always gets sick after taking vitamins for a few days?

YLB asked:

Every time I start my 19-month old on vitamins she gets sick a few days later. It always starts with her throat and tonsils and eventually it leads to her trhowing up and having a horrible cold. Could she be allergic to the vitamins or is this normal? As I said earlier, she starts to get sick about 4 days after taking vitamins. I just thought this was weird.