Feeding and sleeping has changed

So my 2 1/2 month old (1 month adjusted, she was a premie) was getting around 5 ozs a feeding, and sleeping between each feeding. The past week and half everything has changed. She’ll only take 1-2 ozs a feeding after she doesn’t want anymore or falls asleep. She also now only sleeps 30-45 mins at a time before she decides to wake up and eat the rest. She has been extremely fussy and constipated. We have to give her an oz of apple juice a day and that will hardly help. She doesn’t have a temperature and now she has a really hard tome falling asleep, is this normal for her feedings and sleeping amount to change so drastically?