According to Schefter, Callaway's new agent Malki Kawa, who didn't represent Callaway during the combine, said it was for a diluted sample.
A diluted sample is treated as a positive test result.
Callaway has had previous off-the-field incidents at Florida.
He was suspended from team activities during the spring semester in 2016 after a woman accused him of sexual assault. Callaway was found not responsible at a hearing last August and placed back on the team.
Callaway was one of nine Florida players who allegedly used stolen credit card information to fund bookstore accounts this past fall.
He was cited for marijuana possession in May last year and pleaded no contest to possession of drug paraphernalia.