The only logical answer is that your phone has been possessed by the devil. The reason it gets hot is because the that's the way the devil likes it in there, as you can imagine. He then attempts to spread his devilish propaganda by responding to your text messages, but fortunately most mortals don't understand his gibberish devil language. I suggest submerging it in holy water for 3 days*.
In all seriousness, the phone will get hot if it's running processes or using a lot of data. It sounds like something in your phone has gone "rouge". Have you tried simply rebooting it to see if that fixes it? I'm honestly not sure what to make of the text messages thing. If rebooting didn't work, I'd try to pin down any new apps or check battery usage for any apps running higher than normal...and if that didn't give any conclusive info, I'd probably do a factory reset.
*do not submerge your phone in water of any kind, ever.