Is this happening frequently? Does it happen only when watching the movie? Is anything else running in the background. Is the iPhone also charging? What kind of case is it fitted in?
There are many factors which can cause the phone to heat up: it is not always indicative that the battery's health is poor.
But you can check the health of your battery by going to Settings > Battery > Battery Health.
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."
Read this article to learn more about battery health. iPhone Battery and Performance - Apple Support
Continue to monitor the health of your battery and stop using your phone if it heat up too much.