There some other ways of saying "I'm sick". Let's learn some.

Idioms and Phrases to say: "I am sick" in English.

If someone is or feels under the weather, they feel ill: 


Example: I'm feeling a bit under the weather - I think I'm getting a cold.

I'm under the weather. 
or 
I feel under the weather.

1. to be slightly ill
2. to be slightly unhappy.



Example: She's been feeling nauseous and generally out of sorts.

(British English) 
If someone is poorly, they are ill.

Example: You look very poorly. Are you OK?

We use this, to say that we have become sick. We use the word "something" when we are not sure what illness we have. Otherwise we can replace "something" with the name of the illness.

I have come down with a cold.

I have come down with something. 
or
I think I'm down with something.

For showing that you're sick. 

Example: I feel terrible. I think I have the flu.