What is making my S6 overheat?

Hi,

I've had my S6 (SM-G920m) for a little over a year. I've had problems with it overheating, to the point that the camera won't work (it comes up with an error message) and the screen brightness flickers. It's also becoming increasingly laggy, which I'm putting down to overheating too. I've factory reset the phone and stopped using Nova Launcher, using Google Now instead, but it's made no difference.

In case it helps, I've copied a list of apps on my phone below- I regularly close all apps so only a few will be running at a time. The only reason for the overheating I can think of is that I keep my phone in a sturdy Otterbox case (the Strada) - could this be the problem?

Audible
com.audible.application
Authenticator
com.google.android.apps.authenticator2
BBC iPlayer
bbc.iplayer.android
BBC iPlayer Radio
uk.co.bbc.android.iplayerradio
BBC Media Player
air.uk.co.bbc.android.mediaplayer
Bluetooth Auto Connect
org.myklos.btautoconnect
Boots
com.boots.flagship.android
Calendar
com.google.android.calendar
Duolingo
com.duolingo
Egg, Inc.
com.auxbrain.egginc
Good Pizza
com.tapblaze.pizzabusiness
Google Cast
com.google.android.apps.chromecast.app
Google Now Launcher
com.google.android.launcher
Google PDF Viewer
com.google.android.apps.pdfviewer
LastPass
com.lastpass.lpandroid
List My Apps
de.onyxbits.listmyapps
Lost Journey
me.dreamsky.lostjourney2
m-tickets
net.corethree.lothianbuses
Netflix
com.netflix.mediaclient
Pocket Casts
au.com.shiftyjelly.pocketcasts
Reigns
com.devolver.reigns
Relax M. P.
ipnossoft.rma.premium
Ridiculous Fishing
com.vlambeer.RidiculousFishing
Shazam
com.shazam.android
Sheets
com.google.android.apps.docs.editors.sheets
Sky+
com.bskyb.android.skyplus
Spotify
com.spotify.music
SwiftKey Keyboard
com.touchtype.swiftkey
Tapet
com.sharpregion.tapet
There You Go
com.octogear.thereyougo
Three
com.hutchison3g.planet3
Tiny Tower
com.nimblebit.tinytower
Todoist
com.todoist
Transport for Edinburgh
com.lothianbuses.lothianbuses
TripAdvisor
com.tripadvisor.tripadvisor
Twitter
com.twitter.android
Waitrose QC
com.waitrose.WaitroseQuickCheck

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