Fabricio Werdum appears to have been blindsided by the first failed drug test of his career.

The UFC today announced that Werdum, a former heavyweight champion, was notified today of a potential anti-doping violation by the U.S. Anti-Doping Agency. His potential violation stems from an out-of-competition sample collected April 25, slightly over one month after his knockout loss to Alexander Volkov (30-6 MMA, 4-0 UFC) at UFC Fight Night 127.

In his first public address, Werdum, 40, called the situation a “misunderstanding” on Instagram.

Hey guys, I got up this morning to the news that one of my urine samples from April tested positive for a prohibited substance. I am working with my team, the UFC and USADA to understand what happened. I’ve always been careful with everything I take and I’ve always supported a clean sport. We will work hard to solve this misunderstanding and I hope soon to be able to go back to the octagon and do what I love

Werdum (23-8-1 MMA, 11-5 UFC) reportedly was set to face Aleksei Oliynyk (56-11-1 MMA, 5-2 UFC) at UFC Fight Night 136 on Sept. 15 in Moscow. In light of USADA’s notification, it’s unlikely those plans will hold. USADA will now oversee the results management process before making a final ruling on Werdum’s case.

Werdum’s loss to Volkov snapped a two-fight winning streak. On a quest to recapture the UFC heavyweight title he lost to current champ Stipe Miocic, Werdum has gone 3-2 in the octagon since then.

For more on the UFC’s upcoming schedule, check out the UFC Rumors section of the site.