I would imagine that the doctor probably meant you had an electrolyte imbalance, since ES will remove sodium, potassium, etc, along with the water it draws to your intestine. The first successful flush I took waaay too much and I got pressure in the head and arms, dizziness, the "not enough salt" feeling. The "too much salt" feeling would be similar, but with tingling and a feeling like something is trying to suck the liquid out of your head and arms (eew!). I can't even take one tablespoon of the stuff without getting a lot of discomfort, but I always keep a few electolyte drinks around during the flush, I find about 24 ounces of Gator Aid smooths things out.
Potassium I found is real important, as it helps your kidneys to regulate your electrolyte levels.