HI Well come to HCM Your concern is appreciated, the baby is just 3.5 months old and this is the period where great changes can be seen as long as weight is concern and it is the normal phenomenon, the only thing that mother needs to be worry about the ill condition of baby, like fever, diarrhea, vomiting, respiratory symptoms, moreover if the pediatrician s opinion is okay ans said it is nothing to worry then no needs to worry about this, no question is there for hidden disease, even no need to give any medicine without any indication of illness, breast feeding is best for baby hope this information helps.