I have been dealing with food sensitivities for many years. After many rounds of Antibiotics last year, I have a confirmed case of candida overgrowth and can say with conviction that that the sensitivities are more numerous and the reactions more dramatic. (There are reasons for this...leaky gut syndrome, hyper immune response...) Heart palpitations, racing, a feeling like your heart is skipping around....could definately be signs of food allergy or sensitivity. (Also signs of not being well hydrated.)Because my weight is so low, managing my sensitivities while I'm battling this candida is very important for me. My point is, they are 'managable'. Have you been tested clinically? Have you tried an elimination diet? You eliminate suspect foods, ususually the ones you crave or eat everyday, for a period of time and one by one add those foods back into your diet at 4 day intervals. It's interesting that you mention boxed brownie mix. With packaged foods it could be one or more of the obvious ingredients or, very likely, an additive such as artificial color, flavor or preservative.
I didn't mean for this to go on so long.....My best to you...sometimes it benefits us greatly if we become our own sleuths!