The fast onset is concerning but could point to it not being the device/battery. This device is approaching 3 years old but typically with normal use most devices may see some fall off but gradually over time. Sudden or drastically shortened battery life will often point to some change but not always.
Have you downloaded any new apps, including battery savers, data savers, or anything affecting connection?
Have you been in areas that have low data signal to the device?
Has there been a change to your schedule?
New apps that run in the background can affect your battery life. Even if the app doesn't run in the background a new game app for instance can affect your battery life just by way of you playing on the device more or for longer. Besure to check your Screen On Time. Checking your battery usage page in settings can reveal what apps are using the most battery. You can use this list to make decisions about how to address high usage app. Battery, Data or even Memory saver apps can have adverse effects and consume higher levels of battery even if they are supposed to save battery. This is because these apps often run in the background, close or shutdown apps that the system feels are needed causing them to have to reopen. This extra work uses more battery.
Being in low signal areas can affect your battery life because the phone will try to establish or maintain a connection and this can cause an increase in battery use. To combat the effects of being in a low signal area Airplane Mode can be used to prevent the phone from constantly searching.
A change in your schedule can affect your battery usage as many devices attempt to learn the users patter of use and adjust when certain actions happen. It can take a device days or even weeks to learn a user and sudden or large changes to the schedule can throw this pattern off. You wouldn't expect a huge change from this but it may feel big if you're used to one thing but notice that you're getting something else.
Let us know what you find and we'll continue to offer advice but without feedback from you we're left guessing.