I went three months without sugar while dieting. That ended at a birthday party, where I had a tiny piece of chocolate cake. The cake immediately made me ill. I started sweating and became dizzy. After that I was convinced that sugar is awful for our bodies.
Honestly, I've heard similar effects from just about everyone whose avoided any category of food for an extended time and then reintroduced a food that was in the avoided category (whether it's meat, sugar, or any of a number of other things.)
The effect you describe may be more a sign that eating things the body isn't prepared for are disruptive, rather than the harmfulness of the particular avoided food. (Even where the avoided food actually is harmful.)
After five-ish years of mostly sticking to Paleo I have a similar reaction (heart palpitations + general awful feeling) when I eat something like pizza or drink a beer. Not an issue when I eat fries or other simple carb cheats, but it's enough to remind me why I gave up this stuff in the first place.
