I have a headache.

我头很疼。

I have a terrible headache.

我很严重的头痛。

Let me see. Can I touch your forehead?

我看看，我可以摸一下你的额头吗？

Do you feel a lot of pain.

你很痛吗？

I'll give you some medicine. Let's see.

我拿药给你。试试看。

I have a stomachache, Mom.

妈，我肚子痛。

I think I'm going to throw up.

我想我要吐了。

I have diarrhea.

我拉肚子了。

It looks like you got stomach upset.

看起来你闹肚子了。