Subject: Why does body temperature initially decrease with exercise?

I teach year 12 biology.
On numerous occasions, I have checked oral temperatures of students before and 
immediately after say five minutes of vigourous exercise.  Almost invariably, 
the (oral)body temperature is lower after the exercise.  After a short period, 
it returns to colse to the original.  Could it be due to mouth breathing?  

The issue arises in a discussion of homeostasis.  The theory goes like this: 
Exercise increases heat production, so body temp rises, so sweating rate 
increases.  However, it is difficult to convince students of this when they 
measure a lower body temperature.   

Re: Why does body temperature initially decrease with exercise?