Background apps

Not very likely unless one of them has gone rogue. Apps running in the background will usually be music apps and you can pause the music and there are some location apps that can cause this. If you are using a foreground app that relies on internet and cell data, and you have a weak connection it will cause the phone to heat up.

Go to Settings > Battery and see if it shows any apps/processes that are using an inordinate amount of battery. If say you look there and Facebook is using 55% of your battery but you haven't used it much, and you aren't currently using it, place your finger on the bottom left of the screen, swipe up towards the center and then hold your finger there. The Recently Used apps list will appear and you can swipe up on FB or any other app that is using an inordinate amount of battery.

Very few actually do that, what apps are you referring to?