Why does my galaxy s7 get so hot

My galaxy s7 gets hot while doing simple things one time it got burning hot while watching a YouTube and it will get game playing hot while just surfing the Internet and one night I had it charging on my bed kind of under my pillow and I woke up it was so hot the message was up and then these red squares kept popping up i restarted it afterwards but it didn't happend any more I can't tell if this was because it wasn't getting enough air because of it being under the pillow... fast charging wasn't on and it gets really hot while on snap chat.... I've had it for a couple of months and it started doing this less than a month ago

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