This seems to be a common worry amongst the people who begin to wirelessly charge their smartphones. The issue of the smartphone becoming warm during or after the charging becomes complete. These same people who worry about this need to ask themselves if their smartphone is warm after they charge it via a wired connection. Guess what, this is a normal occurrence when it comes to charging devices. Just like when you use a video game console or computer for an extended time, the motor will begin to heat up.
Do not fret people, this is ordinary. Now, if you notice that your smartphone is scorching hot to the point where it is impossible to touch, then you could have a problem on your hands. To figure out if it is the charger or your battery, test it with a wired connection. If it remains scorching hot, it is simply your battery.