I'm no doctor, just a dumb Hapkido teacher, but if you are eating a lot of carbs routinely like ice cream (and hey when you are young and full of piss and vinegar why not) then your blood chemistry is spiked constantly with insulin because your body is just trying to keep you alive. Your body gets adapted to that quick easy sugar as food and is happy to convert it to fat but seldom needs to convert the fat back into glucose for cellular function. Thus when you cut the sugar off you feel like crap because you aren't adapted to using fat for fuel.
Just so you know, during a fast you won't always feel very good. Autophagy in particular can give you headaches, nausea, and other not fun feelings. If your body encapsulated a toxin ten years ago (and it probably made you feel like crap then) and then autophagy opens that encapsulated toxin up again to get rid of it, you get to experience the toxin all over again.