since two days my almost 4 month baby started sucking her thumb.and since she is taking way less feed. she is a bottle fed baby

A. Babies have natural rooting and sucking reflexes, which can cause them to put their thumbs or fingers into their mouths — sometimes even before birth. Because thumb sucking makes babies feel secure, some babies might eventually develop a habit of thumb sucking when they're in need of soothing or going to sleep.