Hot spots on the human body caused by cell phones are caused by the EMF/RF (ElectroMagnetic Frequency/Radio Frequency) emitted from the phones (or any mobile wireless device). (There is much conjecture about the safety of these frequencies and whatnot... That is not what I am here to talk about).
In this case, the term hot spot does not refer to you being a WiFi hot spot! It simply means that since the frequencies are similar to that of a microwave, you will absorb them and they will cause heating internally. These areas will remain warm for a time - whether you can feel it or not, some people can, some can't. Shown here is a thermal image of someone who recently used a cell phone and one who had not.