Your body is designed to react quickly to fear, sending out powerful hormones and signals to various body systems to give you the energy to run or the power to fight. In some people, the fear impulse goes awry, and coping mechanisms become unmanageable, resulting in a disorder that causes the body to feel the side effects of fear even when there is nothing to fear. However, even when no anxiety disorder is present, these processes will cause unpleasant physical side effects as your body works in survival mode.
Diarrhea and the sudden need to empty your bladder are both side effects of fear. This is caused by a combination of all your body systems going into overdrive and moving more quickly than normal, as well as your body's need to lighten itself to make running easier. Additionally, your body sends blood away from the digestive tract, causing it to spasm, resulting in diarrhea.
Sweating is your body's common reaction to fear. It's caused because adrenaline brings blood to the skin's surface, causing the pores to open and allowing sweat to pour out. This is in response to your body's natural desire to run from dangerous situations. An article in "Psychosomatics," the magazine of The Academy of Psychosomatic Medicine, suggests that fleeing is your body's response to fear, occurring before the fight response. Sweating facilitates that response, whether or not you actually flee.
You may describe the rapid heart rate associated with fear as feeling like your heart is going to explode; but, in fact, your heart is just working diligently to provide as much blood as possible to help your body deal with the fear. An article at ChangingMinds.org reports that the amount of blood pumped by your heart increases fivefold during the fear response.
Weakness is sometimes a fear response, causing your body to be unable to move or to move only with wobbly knees. This is because of the "fright" response, which causes your body to want to play dead in an effort to make it unappetizing to a predator. Weakness will usually occur only after flight and fight responses are in place and your body has perceived them to not be working.