Your battery is definitely due for a replacement.
Although iPhone Battery and Performance - Apple Support doesn't specifically address "swelling," it states:
"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."
IMHO, if your battery is actually "swelling," stop using the iPhone.
If your battery is not "swelling," the referenced Support Article also states:
"If your device performance has been impacted by an aged battery and you would like to get help with a battery replacement, contact Apple Support for service options."
Using that link will offer a mail in battery replacement.