Dawn Goodacre said she regularly slept with nine-week-old Henry Whitehouse nuzzled in her shoulder and would wake to breastfeed him at regular intervals.
He did not regain consciousness on 20 December and died at Northampton General Hospital.
Coroner Anne Pember recorded an open verdict and repeated her warning that co-sleeping is unsafe.
It comes just two weeks after the inquest of 11-week-old Darcie-Rose Souster, who was starved of oxygen while sleeping on her father's arm at home in Wellingborough.
Ms Goodacre sobbed throughout her evidence at Northampton County Hall.
"I went to kiss him on the head and I realised his temperature wasn't right," she said.
"I screamed to get an ambulance."
He was taken to Northampton General Hospital but did not regain consciousness and died.
Henry's father, Kieron Whitehouse, said he and Ms Goodacre were "both in shock and devastated".
In a statement read to the court, he said Henry was Dawn's fifth child and while he was unexpected - Ms Goodacre hadn't realised she was pregnant until she gave birth - he was well cared for and much loved.
Consultant paediatric pathologist Dr Roger Malcomson, who oversaw the post-mortem examination, said the cause of death was unascertained but linked to sudden unexplained death in infancy and "in the context" of co-sleeping with parents.
Mrs Pember said she wanted to alert all parents that "to sleep with a baby is an unsafe sleeping environment whether on a bed or a sofa."

The grant is being cut from Â£6.7m to Â£5m by 2020, but most of S4C's funding now comes from the BBC licence fee.
The debate, in the Commons, will be led Carmarthen West and South Pembrokeshire Conservative MP Simon Hart.
In December, Broadcasting Minister Ed Vaizey said S4C was well funded, with a "guaranteed income" of Â£90m a year.

The action will take place on 25 January and 1 February and affect Neath Port Talbot Hospital, Morriston in Swansea and Princess of Wales in Bridgend.
Unison said non-emergency surgery was "likely to be severely disrupted".
The health board said it was fully committed to resolving concerns.
Unison said affected members at Abertawe Bro Morgannwg University Health Board wanted "pay parity with colleagues doing the same job in other hospitals in Wales".
Earlier this month, 54 staff members voted in favour of taking the industrial action, with two voting no, the turnout for the ballot was 78.9%.
Mark Turner, Unison organiser for ABMU, added: "They are highly skilled technicians of many years training yet are treated as though they have just walked in off the street.
"They are extremely disappointed that after two years of discussions the health board continues to ignore their very reasonable claim in the hope they will go away."
The health board said all staff were valued and it had been working with the trade union to develop a new job description.
The board said the current service needed a workforce with a mixture of skills at different bands.
"We are sorry our continuing discussions with Unison have not been able to resolve our staff's concerns," a spokesman added.
"We remain fully committed to continuing our partnership work with Unison to resolve these concerns and have offered to involve ACAS to assist with this issue.
"In the meantime, we would like to reassure patients should strike action go ahead we will be working to ensure any disruption to services are minimal."
A similar dispute involving X-ray department workers has been resolved.
A baby sleeping with his mum on a sofa was found cold and unresponsive when she woke up, an inquest has heard.
[NEXT_CONCEPT]
MPs will debate later how Welsh language TV channel S4C should be funded in the future, as its grant from UK ministers continues to shrink.
[NEXT_CONCEPT]
Staff in the sterilisation and disinfection units at some hospitals in south Wales will go on strike for two days over equal pay.