UFC's Yancy Medeiros to Drug Tester, I'm in the Middle of Having Sex!

"Having sex in my living room and [the US Anti-Doping Agency] comes pounding on my door at 8pm."

That's UFC fighter Yancy Medeiros explaining the most AWKWARD drug test in his life ... when a tester came pounding on his door in the middle of a banging sesh!

FYI -- USADA often conducts random drug tests on athletes to try and prevent the athletes from cheating. So, they can literally bang on an athlete's door at any moment to demand a urine sample, and the athlete has to deliver the goods!

Seems to be the case with 33-year-old Yancy "The Kid" Medeiros -- who was engaging in some romantic activities over the weekend when a USADA agent came calling.

Yancy says he made the guy wait outside until he finished up with the sex ... but the reaction from the tester was worth it.

"Best part, him trynna be all professional as I look him in the face, knowing this foo heard everything going down," Yancy said.

"Been tested pre/post competition, but 'during' is a first," he joked.

Medeiros (15-7) has been fighting in the UFC since 2013 -- so, he should be pretty familiar with the USADA testing situation.

He's currently on a 3 fight losing streak -- most recently taking an "L" to Lando Vannata back in February -- but the Hawaiian is expected to return to the octagon soon.

Good to see he's staying "active" during his time off though ...