She said she "poured the energy" of the shock into discovering who her birth parents were and eventually located them using her own research.
Cullen went on to adopt one more time, bringing the number of children she has with her husband to six, including her three birth children.
Two are now adults, three are teenagers, and the youngest is nine.
Now aged 52 and a Reform UK politician in the Welsh Parliament, Cullen said her family experience had given her a particular interest in the issue of adoption, which she said was "one of the most important things you can do for children".
"My adoption was a really successful one and I felt we should give that back," she said.
Before she entered politics, Cullen gave up teaching to stay at home with her children, which she described as "one of the most fulfilling experiences".
Before that, she worked as a science teacher and, after her stint as a full-time parent, she completed a master's degree in additional learning needs.
She told BBC Wales' Sunday Supplement in October that her interest in the area was partly shaped by family experience, with three of her children on the autistic spectrum.
She said politics was a constant presence in the family home growing up.
"My mother, being from Tredegar and Aneurin Bevan land, was always very political," she said.
Cullen said her parents were a "fantastic influence", but despite growing up in what she called a Labour-supporting household on a council estate, her political views changed as an adult.
"Friends said you need to look at manifestos because at that point I was not sure," she said.
"I voted Conservative for a while and then I had been watching Nigel Farage and the late Ann Widdecombe in the background.
"When Reform came on the scene, I knew that's where my political allegiances were going to lie."
After being elected to represent the Blaenau Gwent Caerffili Rhymni constituency in Wales' parliament for the first time in May, she said her large family were "very proud" of her.
She said she now hopes to use her role in the Senedd to boost support for people who have experienced adoption.
"I would like to see adoptees having emotional support for a lot longer, and support in locating birth families if they require it," she said.
She added that it was "up to the adoptee how they formulate a relationship with their birth parents".
She also feels that children were "sometimes left in care for too long".
"Adoption is a really important way of showing care, compassion and love and it gives a child a foundation from which they can thrive, learn and grow," she said.
Cullen said she viewed adoption "through the lens of my children and the process I went through with them", and that more care was needed by parents going through it.
"It's not up to scratch. There's a lot to do."
Additional reporting by Adrian Browne
Extracted and lightly reformatted for readability. · Source: pt