Hello,
As per my clinical experience, the persistent 
vomiting after feeding may be related to 
allergy to lactose if there is recent change in the formula feeding. Rest of the causes may be as:
   - Viral infection of the gut
   - Disturbances of GI flora
   - Early 
appendicitis or other intestine pathology
   - Others
In my clinic, after clinical examination, I do recommend:
- Certain dietary care
- Anti emetic as ondancetron or 
metoclopramide syrup
- Complete hemogram evaluation 
- 
Ultrasound of the abdomen and pelvis
- Further management guidelines as per the review of reports
Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. Bhagyesh V. Patel, General Surgeon