This is how I personally would say it, in the most natural, colloquial way possible... "Whenever I have even a bite of pineapple, I seem to always suddenly get really bad stomach cramps." Let me know if you'd like me to explain why I chose to add certain words.

This is how I would say #1: "If I ate a bite of pineapple, I would have stomach cramps all of a sudden." #2 is technically correct, although usually "suddenly" is used with the past tense. "I suddenly got sick..." Some other expressions: Out of nowhere Out of the blue

I would use the word immediatley instead of suddenly in the context that you are using it here. So, for instance, Whenever I have even a bite if pineapple, I immediately get/have stomach cramps. I think to make it souns even more natural for the Western Massachusetts area, I would also change "whenever" to "every time". Whenever is also correct English, of course, so you can still use it. I am only suggesting "every time" because people use that word more frequently than whenever in my area. So the sentence would be, Every time I have even a bite of pineapple, I immediately get stomach cramps. / Every time I have even a bite of pineapple, I immediately have stomach cramps.

This is how I would use "suddenly" and "all of a sudden". 1. We were walking through the woods and suddenly / all of a sudden, a deer jumped out of the thicket. 2. It suddenly began to rain while I was waiting for the bus. / All of a sudden, it began to rain while I was waiting for the bus. 私がバスを待っている間、雨が降りはめた。 3. I was about to stop looking for the dog but then I saw him suddenly / all of a sudden by my friend's car. 私はその犬を探すことを止めるところだったが、偶然その犬が私の友達の車の隣に立っていたことを見た。

Suddenly and all of a sudden are interchangeable.