Officials say that she was unable to make the speech at a Congress party rally in the state of Uttarakhand because she had viral fever.
In August Mrs Gandhi went to the US to receive surgery for an undisclosed medical condition.
Her party has repeatedly refused to comment on the details of her illness.
But media reports have suggested she was treated at a specialist cancer hospital in New York.
The 64-year-old Italian-born leader is seen as India’s most powerful politician. She holds no official post, but many consider her the de facto head of the government.
Correspondents say the Indian government faced serious problems during her month-long absence – particularly in relation to corruption scandals – and would have coped better with them had she been around to give advice.
The decision to withdraw from the rally disappointed up to 20,000 supporters keen to catch a glimpse of Mrs Gandhi as her Congress party gears up for a string of important state elections next year that are likely to be crucial to determining whether it can return to power in federal elections due by 2014.
Mrs Gandhi is the widow of former Prime Minister Rajiv Gandhi. She nominated Manmohan Singh to the prime minister’s post in 2004 but is frequently portrayed as being more powerful than him.
She is at the head of the Nehru-Gandhi dynasty, which has ruled India for most of the time since independence in 1947.
Her son and MP Rahul Gandhi is tipped as a future prime minister