Good luck with the docs appt.
My now 10 month old was getting something a bit similar, though it was fairly clearly a reaction to certain foods and it sounds like the vomiting stage was more severe.
2 hours after he eats egg or sweet potato or fish (and maybe other stuff we don't know about yet) he vomits profusely for a couple of hours till he's absolutely empty. The last couple of vomits are just bile. Then he has diarrhea for a few days before it all clears up. At first his stool is almost white, a really pale tan or yellow and then it slowly returns to normal. The explanation I got for the colour is that he's thrown up so much bile the day before and that affects the stool. Once he's accumulated enough bile again, his stool goes back to normal.
I also thought it was gastro the first couple of times.
Are there any newish foods? Has she been having cows milk for long?
Good luck working it out!