A video later emerged of the Democratic presidential nominee in which her "knees buckled" as she left the event.
She was then helped into a van by security staff.
It has now been revealed by her doctor she was diagnosed with pneumonia Friday.
Lisa Bardack, Clinton’s doctor said: “Secretary Clinton has been experiencing a cough related to allergies.
“On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia.
"She was put on antibiotics, and advised to rest and modify her schedule.
“While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely."
-Express UK