I would say tummy bug or tummy ache, my friends have often laughed at me. I asked them what else should I say and they said belly.
I was watching Bear Grylls in Costa Rica last night and he said a tummy bug and I wouldnt have thought anything of it but my friend laughed and said why is he saying tummy bug??
See I'd say Tummy Bug or Belly Ache... but Belly Bug just sounds odd!
I also think Belly makes me think of a big hairy man belly... but tummy sounds more slender! Lol
its a toss up where I come from (birmingham) we say both belly and tummy, I have always said tummy as opposed to belly, belly reminds me of the ad where the belly chases the man and falls of the bridge (chanting "bellies gonna get yaa")