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.

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.”

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 later … but in the videos we have of her at the hotel, she’s slouched in a wheelchair around 12:30 AM Sunday and is on a stretcher being loaded into an ambulance just before 1 AM.

She estimated her drink was allegedly spiked about 30 minutes after she got to the bar … telling us she left the glass at the bar to go outside for a smoke. When she came back, she says there was a napkin covering her glass — she says she didn’t put it there — and she removed the napkin and drank her wine.

Tara’s had well-documented substance abuse issues in the past … but she insists she only had one glass of wine at the hotel before she woke up in a hospital bed.

Worth noting … folks who were there tell us in their limited interaction with Tara at the hotel bar, she seemed fine, but things went downhill fast — and her behavior changed rapidly after the bar closed up and folks took the party out into the lobby.

Tara mentions there were a bunch of YouTubers in the hotel filming when she was there … and she says she’s considering filing a police report to get to the bottom of what happened.