Why does the heart slow down when individuals have diarrhea?

It is called a vagal response and Sooozi description is right on. When we bear down the heart slows. Even though it may not seem that we are bearing down with diarrhea we are still pushing with our rectums.

Could you be thinking of vasovagal syndrome? In Wikipedia it is described as follows and can be triggered by many, many things. Look there for more information that may be helpful. Dehydration is one of the many causes...

People with vasovagal response are typically recurrent, usually happens when the person is exposed to a specific trigger. Prior to losing consciousness, the individual frequently experiences early signs of symptoms such as lightheaded, nausea, the feeling of being extremely hot (accompanied by sweating), ringing in the ears (tinnitus), an uncomfortable feeling in the heart, fuzzy thoughts, a slight inability to speak/form words (sometimes combined with mild stuttering), weakness and visual disturbances such as lights seeming too bright, fuzzy or tunnel vision, and a feeling of nervousness can occur as well. The symptoms last for a few seconds before the loss of consciousness (if it is lost), which typically happens when the person is sitting up or standing. When sufferers pass out, they fall down (unless this is impeded); and when in this position, effective blood flows to the brain is immediately restored, allowing the person to regain consciousness. Fainting occurs when the lost of oxygen to the brain