650 to 700ml feed is adequate for a 3 and half month baby.Make sure baby feeds well.
A baby doubles it's birth weight by 5 months of age,in your case the baby has already gained it in 3 and a half month.so there's nothing to worry about weight gain as of now.
Next Steps
keep checking weight weekly.Consult a paediatrician if weight remains the same in next week also.
Health Tips
Mother's milk is best for babies.it's important to breastfeed.