Q: Iphone 6s plus heated up suddenly.

Hello s60rp10n_o7,

Thanks for using the Apple Support Communities!

From your question, I understand that your iPhone has become warmer than you expect. This can be normal and expected behavior, depending on what the iPhone is doing. The following article has more information on this. 

When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

Cheers!