元気じゃない

There is a nice English idiom to say we are not well:

I'm feeling under the weather.

When you are asked "How are you?" and the answer is negative, you can reply "I'm feeling under the weather." (= I'm not well.)

We can also use this idiom for other people:

"How's your husband?"

"He's a bit under the weather; I think he's been working too hard."

"How's Sachiko?"

"She's been under the weather recently due to a cold."