A sudden change in a newborn's usual pattern, especially being harder to rouse, is something doctors really need to assess in person rather than us guessing from the outside. On one hand it could be nothing, but on the other hand a 1-month-old sleeping unusually deeply after a feed change is the kind of thing that is better checked sooner than later, so please bring your LO to the doctor or A&E now and don't wait to see if it improves.