OnlyFans star Lily Phillips is rethinking her plan to screw 1,000 dudes in 24 hours ... telling us U.S. Customs and Border Protection recently threw a wrench in her ambitious sexcapade.

Lily tells TMZ she got detained at the Las Vegas airport last week after authorities recognized her ... and straight-up told her that if she went through with her "work," she'd never set foot on U.S. soil again.

The Brit tells us she was mainly in town for the AVN porn awards ... and she says she did plan to pull off the stunt to coincide with the event. But, after getting warned about messing with her tourist visa, she ditched the "work" and just had fun at the show instead.

This isn't the first time, though -- Lily tells us she's gotten grief from U.S. border security before about "working" -- so she wasn't shocked, but she was irritated about being detained over it for hours this time.

Looks like it worked out in the end, though -- especially since the 1K guys in 24 hours record was recently broken by fellow OnlyFans star Bonnie Blue. So Lily had to adjust her plans ... but she didn't spill the beans to TMZ about what she has in mind instead.

Whatever it is, Lily's taking it back to her home turf in the UK -- and she's promising it'll be one well-oiled event, as she's learned from her "mistakes" in the 100 guys scenario -- mainly, going big on buckets of lube.

We've reached out to the U.S. CBP ... so far, no word back.