Rosie O'Donnell -- I Had a Secret Heart Attack Last Week

Rosie O'Donnell suffered a major heart attack last week but kept it a secret ... until now, when she revealed the whole scary ordeal on her blog ... as a poem.

In a post on Rosie's official blog dated August 20 ... the comedian drops a bombshell with an entry titled "My Heart Attack" ... in which she explains how she went into cardiac arrest and didn't even realize it.

In the post she explains (in poem form) ... she experienced chest pains, clammy skin, and vomiting ... she then took an aspirin -- but never bothered to call 911.

The next day she went to see a cardiologist and was informed by her doctor that her symptoms were a result of a "widow maker" heart attack.

She says in the post, "50% of women having heart attacks never call 911 ... 200,000 women die of heart attacks every year in the US ... by some miracle I was not one of them."