Olivia Culpo had to throw on her BF Christian McCaffrey's hoodie before her flight to Cabo ... and it wasn't due to cold weather -- it's because the airline told the 2012 Miss Universe she was showing too much skin.
OC's sister, Aurora, documented the whole debacle on Thursday ... showing Olivia in an all-black outfit with biker shorts, a crop top and a long cardigan.
Aurora says OC was called up to the front desk at their gate before boarding ... and one of the American Airlines workers told her she needed a blouse in order to get on the flight.
What's interesting -- the Culpo sisters point out another passenger who was also wearing a crop top, yet that lady wasn't told to cover up.
Luckily for Olivia, she didn't have to run to a nearby souvenir shop to fix the situation -- her NFL superstar boo offered up his comfy hoodie ... which led to a pretty cold and uncomfortable ride for him up in the air.
To make the whole situation even more inconvenient, the group's flight was delayed a bit ... but eventually, they made it up in the sky.
We've reached out to AA for comment ... so far, no word back.