Mr H,
Could you give us some more information? How long has this been going on? Does she have any other symptoms?
The first thing that comes to my mind (only because I had one) is a possible eating disorder. Maybe she's using the stomach ache as an excuse to avoid eating.
I know I'm really jumping to conclusions without hearing more about her, so I don't want to alarm you. You're right to be concerned though.
Does she have any other strange behaviors with food, like making elaborate meals for other people but never eating any herself (saying she ate enough while cooking, stomach ache, etc.)? When she does eat, does she cut it up in tiny little pieces (I could make a meal out of a single green bean), sabotage her food by "accidently" spilling it, finding a hair in it, or doing anything else not to eat it?
Even if it's not an eating disorder, no one should have to suffer with constant stomach aches. Please tell us more!