Pain in the abdomen (stomach ache or abdominal pain); can range from mild discomfort to severe pain and have various causes.
朝から腹痛がして、学校を休んだ。
I had a stomachache since the morning and missed school.
食べ過ぎで腹痛（はらいた）が起きた。
I got a stomachache from overeating.
腹痛がひどいときは医者に行った方がいい。
If the abdominal pain is severe, it's better to see a doctor.
彼女は急な腹痛でトイレに駆け込んだ。
She dashed into the restroom because of a sudden stomachache.