An allergy is an immune response. Meaning that your body will mistake a foreign substance (food or otherwise) and have an intense reaction to it such as hives, swelling, intestinal upset etc. An intolerance is something that your body can't digest because it is missing a certain element or enzyme to digest it properly. Such as lactose intolerance. That can cause bloating, gas, nausea and sometimes diarrhea. While a milk allergy can cause hives, swelling etc. Have you kept a food diary yet? If not, you may want to do that to see if you can find a common culprit (in case it isn't only sugar) that is causing you grief. It could very well be something that the sugar is with such as with candy bars, it can be preservatives or other ingredients that is affecting you. What type of sugar are we talking about
here? Sometimes the refining process of sugar can cause some affect on people. You may want to give Stevia a try and see if it helps.