Use: iPhone Battery and Performance - Apple Support to analyze your iPhone battery health.
If you find out that...
"Battery health degraded
If battery health has degraded significantly, the below message will also appear:
Your battery’s health is significantly degraded. An Apple Authorized Service Provider can replace the battery to restore full performance and capacity. More about service options…
This message does not indicate a safety issue. Your battery can still be used. However, you may be experiencing more noticeable battery and performance issues. A new replacement battery will improve your experience."
Follow the steps listed in: iPhone Battery & Power - Official Apple Support to get your battery replaced.