WordNet (r) 3.0 (2006):

stomachache
    n 1: an ache localized in the stomach or abdominal region [syn:
         stomachache, stomach ache, bellyache, gastralgia]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "stomachache":
   ache, aching, angina, backache, bellyache, cephalalgia, colic,
   collywobbles, distress, earache, fret, gnawing, gripe, gripes,
   gut-ache, headache, heartburn, hemicrania, megrim, migraine,
   misery, odontalgia, otalgia, pyrosis, sick headache,
   splitting headache, throbbing pain, toothache