Asked about the incident this morning on "Fox and Friends," Trump said, "I think it's an issue" in the campaign.
"Something is going on, but I hope she gets well," he added.
Asked by the hosts whether someone was expected to replace her as his opponent as a result of the illness, Trump rejected that possibility.
"I don't think they'll replace her," he said.
Trump said he has taken tests related to his health and will be releasing "specific numbers." He said the reports will be finished later this week.
Regarding the comments Clinton made at a fundraiser this weekend that half his supporters are a "basket of deplorables," Trump said that it showed a "lack of respect" and that he found it hard to believe she would say such a thing.
Clinton's remarks were not without precedent. She addressed Trump's alt-right support in a speech this August, saying he built his campaign on "prejudice and paranoia."
Trump called the "basket of deplorables" remark the "biggest mistake of the political season" and repeatedly said Clinton showed "anger" when she said it.