First boot up, warm phone?

[fashofasho]

Technically it is not the first boot up since the store employee started it for me to show me that the phone works. But when I went home, I turned it on for the first time myself and noticed after about 4 minutes the phone started getting warm. Not warm as in me holding it for a long time but the phone itself. I set the phone down on standby and came back to it later. It has never been that warm ever since. Any ideas on why the first boot up was warm? I was not doing anything crazy nor have the settings changed on my phone to make it "cooler" since then