원래 originally/essentially, -ㄴ/은/는 편이다 to tend to..., -서 because 원래 위장이 좋지 않은 편이*어*서요. Literally: Because essentially my stomach tends to be not good. I think English speakers say just "Because I have a weak stomach".

HI ~ um...simple word.... "I have a bad stomach" xD

I'd say like "because I often get a stomach problem/trouble".^^ As Hailey said above there is no point to translate "원래~편이다" into an English phrase because when it comes to person's habit,characteristics or tendancy, the present tense in English indicates that kind of stuff. The meaning is already in it. ^______^

주변 문장이 더 있으면 좋았을 텐데..