I usually start with painful cramps in my lower belly, I have instant D which mean I better hit the bathroom quick (within seconds if possible) or I won't make it. Then the sweats come, my mouth waters, I feel hot all over and my belly feels like I am going to barf.
There is nothing worse especially if I am not near a bathroom. I have been caught in the car to many times to count and then the race is on to get to a bathroom, preferably at home, before the explosion.