Simply closing apps is not a solution here (on the contrary it is even better to not close apps, or only when they behave unexpectedly. more info)
You might have a wakelock, but I think it’s not probable because the phone only heats up when your mother is talking on the phone.
I first thought about opening the phone up and clearing it of any dust, but this article suggests that you take off covers from your phone, if you have any.