What Can I Do About Frequent Vomiting Leading To Weight Loss In An Infant After Feeding?

Please, Doctor, my 9 month old baby is looking clean, was very chubby till like 4 months old then suddenly started to drop in weight. I dont know what to do now. She weighs 8.2kg now. Can you help ad to what I should do to help her gain weight. The problem I have noticed is, she vomits a lot especially after feeding. Thank you