
I use both sick and ill interchangeably. I have said, "I think I'm going to be ill" when my stomach was about to rebel. . . which it did plenty while carrying each of my 3 sons

. I've also said, "I'm gonna be sick!" When feeling like I was "coming down with something" (another expression for feeling like you're becoming ill or sick) I've said and heard "I feel ill/sick" meaning that I was achey or coughing heavily (like with bronchitis).
Interesting link, tdol. Interesting discussion.
So, in AE, it's interchangeable.
Smiles,

Sweet Momma Sue