15yr old Border Collie/Spitz Mix - Vomiting

[Missy n Malibu]

I am very concerned about my 15yr old baby, she started throwing up her food about 2-3 hours after eating. This has been going on for the last three weeks. I called the vet and he suggested that I change her food to Hills Science Diet for sensitive stomach, so I did. I have two dogs so if I had to change food for one it goes for both because they eat out of each others bowlsl

This is the fourth day, days 1-3 were good no vomiting until tonight. She vomited all of what she ate about 2 hrs after eating. I don't know if she eats to fast or what, she ate a little more tonight than the nights before. She was begging me for more food, I felt bad because she seemed hungry

I only give her about 1 1/2 cups because she weighs about 23lbs and I give my 7 yr old 2-3 cups because she weighs like 70 lbs.

What can I do to help her? I feel so helpless