Johanna Konta says she suffered heart palpitation during her 6-4, 6-4 loss to unseeded Czech Marie Bouzkova in the opening round of the Top Seed Open in Lexington, Kentucky, on Monday.
The third-seeded Briton needed medical attention just minutes into the contest before she felt well enough to continue.
"My heart rate shoots up for no reason, we didn't have a long point, I wasn't gassing in any way in my lungs," the world No. 15 said.
"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."
Konta said it was not the first time she had experienced the problem.
"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 EKG (electrocardiogram) 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."
The Top Seed Open is the first WTA Tour event in the United States since the COVID-19 pandemic shut down the circuit in March.