"I can confirm that USADA is aware of the situation and is currently gathering information in order to determine the appropriate next steps," said USADA spokesperson Ryan Madden.
During Saturday's postfight press conference, Diaz, who suffered a majority decision loss to McGregor at T-Mobile Arena in Las Vegas, openly vaped while answering media questions.
When asked what substance the pen contained, Diaz, 31, said it was CBD oil -- otherwise known as cannabidiol. Cannabidiol is considered a banned substance in-competition, under the classification of cannabinoids.
"CBD oil," Diaz said when asked about the vape. "It helps with the healing process and inflammation, stuff like that. You want to get these before and after a fight, training. It'll make your life a better place."
The "in-competition" window extends to six hours after a UFC fight. Diaz submitted a fight-night drug test prior to the news conference, the results of which were not yet available. The threshold for marijuana metabolites under the UFC program, as well as statutes of the Nevada State Athletic Commission, is 150 nanograms per milliliter (ng/mL).
The USADA could argue Diaz's admission of using CBD oil within a six-hour time frame is a violation of the UFC's anti-doping policy. If the USADA were to rule a violation has occurred, Diaz could face up to a one-year suspension.
Diaz's representatives were not immediately available for comment.
Diaz (19-11) has never failed a drug test. His older brother, UFC welterweight Nick Diaz, recently completed an 18-month suspension for his third marijuana-related offense. Nate Diaz, who fights out of Stockton, California, already has called for a trilogy fight against McGregor.