I have really bad nausea. I can’t eat anything. Also stomach cramps.
我还特别恶心。什么都吃不下。还有胃痉挛。

Nausea
反胃

Nausea is a type of sickness where you feel like you are close to vomiting.
反胃是你感觉自己要吐时的感觉。

Her nausea was so bad that drinking water didn't even help.
她反胃得非常厉害，喝水都没有用。

Nauseous
恶心的

When someone feels nauseous, it means that they have nausea.
当有人觉得恶心，就是说他们在反胃。

If you have nausea, you can say: "I feel nauseous." or "I have really bad nausea."
如果你反胃时可以说“我觉得恶心”，或者“我反胃非常严重”。

Stomach cramps
胃痉挛

Stomach cramps are where it feels like your stomach is being squeezed really tightly.
胃痉挛是你感觉自己的胃都绞在一起。

When she got her period, she would have very painful stomach cramps.
她大姨妈的时候胃痉挛非常严重。

Stomach cramps are a feeling of your stomach being squeezed.
胃痉挛是你觉得自己的胃绞在一起。

Stomach aches
胃痛

A stomach ache is just an overall soreness in your stomach.
胃痛是胃整个很痛。

I'm not surprised he got a stomach ache after eating all of that pizza.
他吃了那么多披萨，怪不得会胃疼。