Ingredient in dairy free junk food making me feel ill

I'm stumped so I thought I'd ask here as you lot are so knowledgeable :)

I have some food sensitivities I've never been able to work out (I think garlic and onion are some of them) but the ones I want your advice with is in junk food.

Not because I eat loads of it, I'm pretty good with eating reasonably healthy.

But when I do I get stomach issues, like excessive gas, nausea, bloating.

I was eating dairy free chocolate before I gave up meat so I know it's not related to that part of it. I went dairy free first.

Its not sugar because I can drink fizzy drinks with no symptoms. And I never had an issue with normal chocolate.

Sweets/candy (like non veggie gummy sweets) DO make me feel ill though, not that I eat them.

I was wondering if anyone knew what ingredient could be making me feel ill. I don't know what to eliminate from my diet.

(I don't eat junk much but today I had some fudge as a treat and I'm so poorly)

:( :(