There are a number of possibilities. What OS and version are you using and you can monitor the battery usage to set what is draining it. Maybe you have the bluetooth or wifi on which you didn’t before any old apps get updated and can change their background and wifi use etc.
it is also important not to discharge and recharge fully as this exhausts the battery.
You can also check the battery status via dialling *#*#66#*#*
Select Service tests > Test single > Battery status check. You can check whilst charging or not.
if you can provide more info it may be possible to find out what is going on.
You can also take screen shots of the usage as in this post although referring to Android 9 it is a good view and action.