Fruit is full of sugar as well as each fruit having its own chemicals which it produces to protect itself. They are called antinutrients.
Here is a page of info about what it is in the food you might be reacting to:
http://www.plantpoisonsandrottenstuf...reference.aspx
Many folks find that eating fruit with something with fat in it, such as cream, makes it easier to digest. I noticed, when I lived in Europe, that fresh berries were usually served with cream or cheese.
The insecticides, fungicides, waxes, etc., used on the plants and fruit could also be a trouble.
I know several who can eat frozen, canned or cooks fruits, but some of the fresh ones cause problems.
This paper at Barry Groves' site, on cooked food, might be of interest:
http://www.second-opinions.co.uk/cooking-1.html
Hope these are a help.