The former singer had an on-off relationship with the X Factor mogul for 20 years, and tells the magazine the announcement that Simon was having a baby with Lauren Silverman blindsided her.
"I was completely and utterly stunned," she tells HELLO! of the moment when Simon telephoned to tell her the news. "I was in such a daze, to the point where I wondered if I was dreaming it.
"He sounded…odd, calm, strange, which threw me. I don't mind admitting all sorts of emotions were running around inside me. He was very concerned about how I would take it, which, on reflection, he didn’t need to be. He didn't owe me that consideration." And she admits she felt very differently to when she found out that Simon was engaged to make-up artist Mezghan Hussainy in 2010.
"That was a curve ball," she admits. "But I suppose there has been a certain amount of comfort for me in my not having a child with Simon because he never wanted to have children at all. That's who he was, so you accepted it.
"Then, to discover that he is going to have a child, only with someone else and they’re building this life together…. All those crazy things were going through my head and it’s not fair and it's not pertinent to Simon and Lauren’s situation and I don’t have a right to feel that way. But I'm being honest."
Sinitta also admits that she was stunned to discover that Simon and Lauren had fallen in love, after holidaying with her and a group of friends for years.
"We knew we were all close friends but I didn't realise he and Lauren were falling love and, when I found out, I will admit I was taken aback and did feel a sense of betrayal," she says.
To read more of the interview with Sinitta, including how the excited parents-to-be announced the sex of their baby, how she and Lauren made peace and her thoughts on becoming the baby's godmother, pick up issue 1295 of HELLO! magazine on newsstands or buy the iPad issue by clicking here.