Vomiting 30 minutes after eating new wet food?

Yesterday morning, I gave my cat Sibylla a small dollop of Weruva Cats in the Kitchen - The Double Dip (Chicken and Beef au jus) straight from the freshly-opened can, and she seemed to really love it. She ate perhaps 1/3 of it, and vomited it up about 30 minutes later, heaving twice after making some very unusual meowing noises. Her vomit was mostly clear or translucent liquid (not foamy, if I remember correctly) and it contained undigested chunks of the food along with a treat (also undigested) she had eaten earlier.

She was back to her usual self afterward and has not shown any lasting effects since. In fact, I put a much smaller amount of the same food (this time, microwaved for a few seconds) in with her usual dry kibble this morning, and she has not vomited up that meal. I had tried other wet foods prior to The Double Dip, which she did not seem to enjoy nearly as much as this.

I called the vet, and the staff told me that I might be changing her diet too quickly or feeding her food that's less than fresh (which is not the case).

Based on these observations, what could have caused the vomiting in my cat? Could it have been that I had upset her stomach by giving her only wet that time instead of mixing it in with her usual food?

My cat with a sensitive system did this with a wet food that had beef in it.  It was the same thing he ate which is very good for him.  He's a kibble addict and getting him to eat wet food is a struggle.  Shortly after eating it he vomited.  Has he ever eaten beef before?  Patches hadn't and I just assumed he couldn't eat beef.

Yes both beef and chicken can sometimes be a problem for cats or it just might be because it is new. It is interesting she did not vomit after she had it again so she might be able to tolerate it. If she likes weruva you might want to try tiki cat which is a similar consistency and they have two different chicken flavours. My cat likes weruva and tiki cat and he also likes the new pride line by natures variety and the pride foods come in novel proteins like lamb and duck which are more digestible to a lot of cats with sensitive stomachs. 

www.catinfo.org has some good tips for transitioning cats used to dry food to wet. 

Thank you for your responses!

@Denice: I don't believe she has eaten beef before, though I am not 100% sure. I am sure I have not fed her anything containing beef, at least as a primary ingredient.

@Franksmom: Thank you for the link. I have actually read Dr. Pierson's nutrition article in detail on her site, and it is helpful, though so far none of the tricks I've tried have worked  . And she only ate a very tiny bit of it the second time around, but most of that meal consisted of her usual dry food, so it could be that the dry food kept her GI system from getting upset.

I've been trying to transition for years and a little wet is all I've been able to do.  My vet who believes in wet has a cat that she has the same issue with as has Jackson Galaxy.  I just finished reading his book.  The little tabby that is at the center of his book was the same way and he believes in a wet diet, actually a raw diet.  If my vet and Jackson Galaxy met their match when it comes to this, I don't feel so bad.  I, like you, haven't given up and I get as much wet food into them as I can.