Why is my phone extremely hot and isn't charging correctly?

Hi guys, thanks for listening.

My iphone 4 has been working perfectly fine until today.. During the morning in school i was outside and my phone began to get super hot.. (It was at 70%)

Not even 2 hours later, it was at 30%, then kept dropping VERY quickly! Until it died, and it was very hot. I tried cooling it off, putting it in my pocket and i just forgot about it.

When i got home from school , i directly put it to charge.

(It was charing for about 2 hours) when i came to check it, it had only gone up to 24%.. i was confused because it never takes that long to charge..

So i unplugged it and pluggged it back in, thinking i might've just plugged it wrong.

2 hours passed (So 4 hours) and it only went up to 31% and it was hot!

I dont know what to do.. is there anything you guys can do to help me to fix this?

Is it a battery problem? Is the phone just too old and it's just broken now? What do i do?