The Agni tatva is one of the five elements of which the body is made. When you recite mantras, the agni tatva increases. And it is necessary to maintain 98.40F temperature in our body. If the temperature increases, then the body will unbalance and will get affected by heat. This happens due to recitation of mantras. Mantras are of fie elements. That is why the temperature increases due to recitation of mantras.
So what? Should the recitation of mantras to be stopped? No. You have got this life for a specific purpose- for God realization. I am not saying that the recitation of mantras to be stopped. But you should maintain your body. The body temperature should not go beyond 98.40F. If it increases, then there will be health problems. This is one of the reasons for health problems. If the temperature increases, then the vayu tatva in the body gets disturbed.