Tara Reid is explaining why she's convinced someone slipped something into her drink that knocked her out ... and she says she's fortunate she wasn't raped.

The "American Pie" star joined us on "TMZ Live" Monday and took us through her harrowing night in Chicago ... she says her drink was spiked at a hotel bar. Play video content

We have video of Tara in a wheelchair in the hotel lobby and on a stretcher on her way to the hospital ... she says folks at the hospital told her she was "drugged."

Thing is, Tara says the explanation she got at the hospital was all very "vague" ... telling us she didn't bother to ask what kind of drug laid her out for several hours.

Tara says she went down to the bar around 10:30 PM Saturday and was in the hospital around midnight, and woke up 8 hours

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