She spent about five hours, and was discharged from the hospital after her checkup was completed.
"The Congress President has come back, she is all right and her medical check-up has been completed," party General Secretary Janardan Dwivedi told PTI.
A senior doctor at AIIMS said, "She had cough...she had headache. She took medicines after which she felt uneasiness in Parliament, that is why she was here. We put all the test required for such diseases and we found her normal, and she has left in her own car talking to all the doctors, and thanking all of us."
Union Health Minister Ghulam Nabi Azad said, "Madam, it looks, had medicines for common cold and headache, as a result of which she had some uneasiness and she had to be brought to AIIMS and now all test have been conducted and she is perfectly fine and has gone home."
Sonia, 67, left the Lok Sabha while the voting was underway on various amendments moved by the Opposition and was rushed to AIIMS. Ironically, Sonia could not be present in the Lower House when the landmark bill was passed.
"There is nothing to worry. She is completely fine", Dwivedi had earlier said. Several leaders of the Congress made a beeline to AIIMS to inquire about Sonia's condition.
Dwivedi said that based on the available information from doctors some medicines did not suit her and problems erupted because of their reactions.
AIIMS hospital sources said Sonia complained of exhaustion and mild chest pain and was taken to the Intensive Care Unit as a precautionary measure before she was shifted to the Cardio-Neuro Centre.
The sources said Sonia underwent some tests including an ECG and is under observation at AIIMS where top doctors of the institute including its director R C Deka are attending on her. Balram Airon, Head of Cardio-Thoracic surgery department, is monitoring Sonia's health condition.
Sonia was stated to be suffering from viral fever since Sunday and was suddenly taken ill after spearheading UPA's ambitious Food Security Bill in the Lok Sabha on Monday.
She was seen leaving Parliament at 8.15 pm and holding the hand of Union minister Selja before boarding a vehicle.
Her son and Congress Vice President Rahul Gandhi also accompanied her.
Security was beefed in and around AIIMS.
In a tweet, Senior BJP leader Sushma Swaraj wished Sonia a speedy recovery.
Doctors earlier examined Sonia at her home on Sunday and Monday, sources said.
Earlier in the day, Gandhi led the party in the debate on the landmark bill describing it as a measure to wipe out hunger from the country.