Or have you received an update? That can use a lot of battery for a few days to a week while the system is setting itself up.
Also check the battery itself. If it's removeable, put it on a flat table and flick a corner. If it spins, it's shot (it's bulging). If it's more than 6 months old and you regularly use it until the phone tells you to recharge it (which is between 5% and 10%) it could be on its last legs. If you discharge it until the phone shuts off regularly it definitely won't last long. The charge point for longest life is 50% (40% doesn't shorten the life much).