Sometimes it is due to exhaustion. Baby is tired of sucking as it requires a lot of energy to suck. So they fell sleep on the breast. It is the same for my baby. I will keep my baby engaged during bf and do burping or change diaper to wake baby up and continue feeding. Usually baby is not full yet. Must make sure he is full and satisfied before letting him to sleep. Or else he will wake up crying for milk.
it's the essence of breastfeeding :) dont worry at all. you should feel happy your baby sleeps after breastfeeding. it means youve done something great for your child as a mummy. cheer up mummy!!
Is your baby overfed from formula? Baby can drained breast faster thou. If baby seems satisfied after a feed then baby won't cry.
most likely the baby get tired and fall asleep after sucking nipple for too long