What causes my iPod to enter a Reboot Loop?

So I just woke up and turned on my phone and for the first few seconds it was fine but then it turned off with the like the gray screen and then the apple appeared and I was like it must have died but then I remembered I had 15% so when it turned back on I had 48% then it did it again shut down and it did it three times and stopped

this iPod is generation 7 I don’t know if that has to do with anything. But it’s also pretty old my sister had it for at least 4 years then gave it to me because mine broke

[Re-Titled by Moderator]