Johanna Konta, was beaten 6-4 6-4 by world number 48 Marie Bouzkova in Kentucky.
The Briton needed medical attention early, 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.
“It was very high when they came out. I was having all the checks. I am as fit as a fiddle but it can happen. It took a while to settle down but then it did and I was able to concentrate on the tennis.”
Johanna Konta had also lost to the Czech Republic’s Bouzkova in Mexico in March in her final match before the tour was suspended because of the COVID-19 pandemic.
After a set back by four hours because of a rain storm and after surviving a break-point scare in the opening game of the match Konta appeared to be struggling with the conditions.
Konta has suffered similar issues in the past, at the 2016 US Open and at the Fed Cup last year. Konta recovered to win both matches.
“We don’t know why it happens: it’s been under stressful situations, in no-stress situations. We just don’t know. It’s been sporadic,” Konta said.
“It’s not ideal. I am going to have another ECG and see what protocols we can take. It’s probably not going to happen again for another year or something. I’m not too worried, no.”