Otter's Rock

I am worried because my cat has now thrown up six times in 24 hours.  She has not been outside in the last week, and I can't find anything inside that she could have gotten into.  Aside from throwing up, she seems normal--she is neither more nor less cuddly than normal, for example, and while she is usually a very sedentary cat she doesn't seem to be any more so than normal.  Is this something I should be worrying about?  Should I be taking her to the vet tomorrow?

The only thing that has changed is her food; about a week ago I switched from Purina Cat Chow Complete to Purina Outdoor Cat Chow.  But a) that was a week ago and b) she's had the Outdoor Chow before; which one I get depends mostly on which one they have in stock, the local grocery store being very small.  I didn't transition her from one to the other (taper off one and gradually add in from the new one), but then I never have before and it wasn't a problem; is that the problem now, or something more serious?