Constant tummy pains

does anyone have this? like, having problems with your tummy like you feel it burning, and then cramps, and then nausea, and then gassy, and then sharp pains. let's say like 90% of the time it just feels blaaaah.
the only thing i could think of is that i work different shifts, like day, night, evening, like i could do all 3 shifts in one week but still, why's it THAT bad?
i always think i'm starting a gastro or something. although i hardly EVER puke or have diarreha (like once or twice a year).
and i don't eat that much junk food, i take it easy on dairy and i haven't eaten meat since i was 15. (i'm 25 now). I'm a lacto-ovo pescetarian to be exact. and i do smoke, i cut down to half a pack not too long ago (i used to smoke almost a pack).
oh yeah and sometimes my back hurts too and it really feels like it's linked, you know?
any ideas, or similarities?
thanks.