Ms Thapa, a Nepalese national living in Basingstoke, had taken her baby to her GP in May after becoming concerned that the blue spot markings had become darker and he had developed new ones.
Her son's existing blue spot - sometimes known as Mongolian blue spot - on several parts of his body had been noted in his medical records, following his birth in November.
Ms Thapa was referred to Basingstoke and North Hampshire Hospital where she was arrested by police on suspicion of actual bodily harm assault and neglect of her baby.
The 29-year-old, who was breastfeeding at the time, spent 20 hours in a cell at Basingstoke Police Station until being released pending a medical report.
"It was hard for me," she said.
"I have never been separated from my baby since his birth.
"At that time I was like a criminal. I stayed overnight in a cell like a criminal.
"Without medical proof, they put me in custody."
Her baby was looked after by the hospital while she was detained and she was permitted to express milk to allow him to be fed.
After her release, a social services home visit determined her son was not in danger.
A series of scans later showed no skeletal injuries and a dermatologist determined the markings were blue spot and not bruises. Police confirmed no assault had taken place.
