Johanna Konta says she felt "lightheaded" after suffering from heart palpitations during her straight-set defeat at the Lexington Open.
Konta, seeded third on her WTA Tour return, was beaten 6-4 6-4 by world number 48 Marie Bouzkova in Kentucky.
The Briton needed medical attention early on but, after consultation with a doctor, was able to return to court.
"My heart rate shoots up for no reason," Konta, 29, said. "I wasn't gassing in any way in my lungs."
She continued: "It actually goes up and stays there so it makes me a bit lightheaded - I just needed to see the doctor and the physio.