Why is my iphone overheating?

My iphone 3GS is a mess. The battery is dying way to fast. Like i may have a few hours without having to recharge it, even when it starts at 100%. And i notice that it gets really hot sometimes when i'm usng it, and even when i'm not using it, it gets warm and hot in my pocket.also, sometimes it shuts off, like it'll be at 18% and then just die. Or it'll be at 35% and shut off and ill turn it back on and it'll be at 33%.is there anything i can do to fix this, is it a battery problem, internal problem, or i don't know what.

There's a massive battery problem… I would just buy a new phone, the iPhone 6 is out in a few month wait for that and go for it

With the first part of your problem, press the circle button(home button, square button, whatever its called) twice, and a little screen at the bottom should pop up with a list of apps. Those are apps that you've recently opened. You have to delete them all like a regular app. It makes the battery last longer. Another way to make the battery last longer is to turn off the bluetooth, and dim the brightness. I think the problem with the battery is, your charging it with the wrong charger! It can break your iphone if your using a different type of charger! An example is, if your using an Ipad charger for it, it can break, but, if your using an Ipod charger with it, it should be fine. I think this has to do with the shutting off with the high battery %'s. So make sure you are using the right charger. With the warmness in your pocket, I'm not too sure what the problem is there. Maybe you leave an app open. Hope I helped.

Try having it updated or if memory is full try deleting some stuff, you can always take it to the apple store to get it replaced

My 3GS battery life is great. Keep in mind talking for an hour or two straight will always use the most batter… So if talking at home I would plug it in while on the phone.