Vomiting after feeding. I request. plz!

Good evening doctor garu, My baby yesterday 4 months completed, since last 16 days he is going loose motion( watery almost 15 times a day) I consult a doctor but motion are not in control..at last doctor asked me to change my breast milk to formula milk that is zerolac powder but I don't do...and also he prescribed me a taxim syrup antibiotic I used the syrup for 5 days now motion is in control...but today when I feed him..he vomited almost more than half big glass of milk.. After that he cried a a lot...again I tried to feed him...he drink my breast milk and he went to sleep... I am much worried what should I do..now again can I take him to clinic or it's ok plz help me doctor garu I request...plz