'Selling Sunset' Christine Quinn Flustered Talking About 'Positive' COVID Test

Christine Quinn has an explanation for why she was on a film set 2 days after missing the "Selling Sunset" reunion due to having COVID ... saying she tested "positive" to go to set.

During a line of questioning from Andy Cohen, Christine told him she tested "positive" the night before and the morning of her poolside shoot with Melissa Gorga ... say what?!?

Christine's clearly flustered by Andy's probing questions as she tries clearing things up, explaining she tested positive to go to set, meaning she tested negative for the virus. This is some serious mental gymnastics.

TMZ broke the story ... Christine was seen filming something with 'RHONJ' star Melissa last month, only 2 days after missing the 'Selling Sunset' reunion, which her reps blamed on a COVID diagnosis.

Christine's co-star, Mary Fitzgerald, told us she thinks the COVID case was BS and was simply a convenient excuse to back out of the reunion show and avoid facing the heat.

But, as Christine told Andy on "What What Happens Live," her contract did not require her to film the reunion show. Still, her COVID timeline doesn't make much sense, even though she says she definitely had the virus.