Bola can be stomach too though so why is stomach ache wrong?

I thought the pattern for this was... Mae bola tost arna i? Why is this Mae bola tost gyda fi?

Stomachache is acceptable as one word (at least, in American English...I understand I'm probably in the minority here). It accepts it but says that there's a typo in the answer.

In NW we just say poen bol.