Overheating

It has a sensor, it will protect the phone if it goes past that heat level. What are you doing with the iPhone when it begins to get warm?

So you are getting the message on the screen that it is overheating?

Both of those apps, especially if you are on a fairly weak cell or WiFi signal, will really heat the phone up. If it gets so hot that no one can touch it you might want to send it in for service.