have they checked you to be ABSOLUTELY certain that you're not having contractions? at 31 weeks, i'd want to rule that out before assuming it's reflux or indigestion.
aside from that, i had HORRID indigestion from water - i could drink just about anything else and be fine, but water was just painful - i had horrible pain up between my breasts though, not near my lower abdominal area.
idk, i'd first rule out labor - then i'd be looking at gallstones and things of a similar nature. if all is ruled out, then you may try out some dif meds for whatever your OB thinks you've got - but i'd def try to see my OB soon.
good luck!