Speaking at a campaign rally in Manheim, Pennsylvania, the Republican presidential hopeful said: "But here's a woman, she's supposed to fight all of these different things and she can't even make it 15 feet to her car. Give me a break. Give me a break."
He then did an impression of Ms Clinton, who stumbled after she felt unwell at a 9/11 memorial service, to cheers from the audience.
After Ms Clinton left the 9/11 memorial early, her doctor said she was suffering from pneumonia.
Mr Trump added: "She's home resting right now, she's getting ready for her next speech, which is going to be about 15 minutes and it's going to be in two or three days."
The Republican has said he plans to be "even nastier" about former president Bill Clinton's affairs to "unnerve" Ms Clinton in the next presidential debate. “She’s nasty, but I can be nastier than she ever can be,” he said.
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