Why do my vitamins make me nauseous?

I have been taking vitamins for over a year now and never had a problem. I ran out and it took me a good month or two before I bought some more and this time around the vitamins make me nauseous. It’s the One a Day vitamins for energy. I tried the same brand different kind, I think the weight smart one, and that also made me sick. I quit taking the One a Day and switched to the kind I give my son. Yes, I’m a Flinstones kid. I am very curious as to why this happened.