Swollen battery for the past few days - MacBook Pro (Retina, 15-inch, Mid 2015)
You should definitely locate a service provider that can run a test today / tomorrow. Remote service is easy for this and quick and consider getting a backup and then take your computer to the Apple Store if they are open in your country.

https://locate.apple.com
https://support.apple.com/15-inch-macbook-pro-battery-recall

This is the battery airbag inflating to prevent a problem like fire or failure. Protect your data immediately, the hardware also needs attention. You may be covered by a safety program about the Apple batteries in the MacBook and it has precautions to take. In the past, these original batteries have exploded. I would take it to Apple Store immediately.
Also if the battery continues to swell it will eventually break something inside your computer. Others have mentioned that a swollen battery broke the trackpad on their MacBook Pro. All the more reason to take it to an Apple Store even if you think you are out of warranty. This design is like crumple zones in a car - the hardware is designed to save life in the event the design tolerances are exceeded. If the car is going to crash, it might as well do so safely. Same with a battery, if it’s getting over charged, it might as well let you know clearly and safely.
I also found this fairly recent (2019) article about Lithium Polymer batteries and off-gassing.