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Women’s professional tennis star Iga Swiatek has accepted a one-month suspension after testing positive for the banned substance trimetazidine, the International Tennis Integrity Agency announced Thursday.

Five-time Grand Slam champion failed an out-of-competition drug test in August. She formally admitted the anti-doping rule violation Wednesday and accepted her penalty the next day, according to ESPN.

The ITIA accepted Swiatek’s explanation that the test result was caused by the contamination of a nonprescription medication, melatonin, that she was taking for issues with jet lag and sleeping, the sports news outlet also reports. 

Trimetazidinea is heart medication known as TMZ.

“It was a blow for me, I was shocked and this whole situation made me very anxious,” she said in an Instagram video about testing positive. “At first I couldn’t understand how that was even possible and where it came from.”

The agency said in a statement that it has accepted the 23-year-old Polish athlete’s explanation. 

“The ITIA accepted that the positive test was caused by the contamination of a regulated non-prescription medication (melatonin), manufactured and sold in Poland that the player had been taking for jet lag and sleep issues, and that the violation was therefore not intentional,” the ITIA said in a statement on Thursday. 

Swiatek had previously been suspended in September. That suspension ended on Oct. 4, according to CNN.