Eek!
Dunno what it could be, I'm no doctor... but perhaps its hunger? I get bad pains when my tummy has nothing in it to "work on". Could also be wind... trapped air is horrifically painful!
If u feel up to it, try having a shake and some water... and try a peppermint tea as this is calming and helps soothe ur gut.
Hope u feel better and if it gets no better call 08 45 46 47 and ask a nurse.
No idea what the symptoms of gallstones are, but they can happen when people lose weight...?
Spots xx