Why is my battery swollen? This keeps making my phone shut down.

Hi @sevarinpaul

A swollen battery is usually caused by a faulty/failing battery.

Swollen Lithium batteries can be dangerous and can start fires if handled incorrectly.

Do not charge or use the phone until the battery has been replaced would be the safest option.

Here's a link that in general describes how to handle and to responsibly dispose of a swollen battery.

Here's a video that shows how to replace the battery in your phone.

Search online for BL-58CX battery to find suppliers that suit you best.