I've been feeling ill since last week, so I am sorry that I can't go to your party.

我从上周开始就感觉不适，所以很抱歉我不能参加你的聚会。

youdao

I should have gone this morning but I was feeling a bit ill.

今早我本应该去的，可是我感到有点不舒服。

《柯林斯英汉双解大词典》

For example, under the weather is an idiom meaning ill. So when people say "I'm under the weather", they are saying that they're not feeling well.

例如，“under the weather” 这个习语的意思是“生病了”。所以当人们说 “I'm under the weather” 时，他们是在说他们感觉不舒服。

youdao

But in a statement on Villa's website on Wednesday, Houllier sought to clear up any ill-feeling, suggesting his comments had been misconstrued.

BBC: Gerard Houllier explains Liverpool remark to Villa fans

Ill-feeling between Egypt and Algeria rose in the aftermath of the Algerian football team's victory in a qualification match for next year's World Cup.

ECONOMIST: Politics this week

Irish may face former fly-half Shane Geraghty, who left the Exiles in 2009, but Casey said there is no lingering ill-feeling towards their former team-mate.

BBC: Few weaknesses in Northampton squad - Bob Casey