What you describe is not dumping. It sounds like you ate too fast, you probably didn't chew well enough, and your pouch was mildly blocked. The dry-heaving is you trying to get rid of the blockage.
Dumping occurs after you've eaten. It is your bodies response to eating too much sugar or fat in some cases. Your stomach / intestines put out a "We need water here" which your body responds dramatically. All this fluid in your intestines are what cause the Dumping symptoms. The symptoms include sweating, heart palpitations, nausea, gastric distress, cramps, and the forever popular diarrhea.
Good luck.
Tek