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."
A baby sleeping with his mum on a sofa was found cold and unresponsive when she woke up, an inquest has heard.