Sinitta felt "betrayed" by Simon Cowell when she found out he was in love with Lauren Silverman.
The 49-year-old star, who dated Simon in the 1980s, regularly holidays with her ex-boyfriend and his fellow former flames Jackie St Clair and Mezhgan Hussainy as well as Lauren Silverman - who is pregnant with his first child - but she never expected him to fall for the brunette beauty.
She said: "We knew we were all close friends but I didn't realise he and Lauren were falling in love and, when I found out, I will admit I was taken aback and did feel a sense of betrayal."
The 'So Macho' hitmaker is still good friends with Simon - who is godfather to her two adopted children, Magdalena, eight, and six-year-old Zac - but admits she was in disbelief when he told her he was expecting his first child with the New York socialite because he had previously said he never wanted kids.
She explained: "I was completely and utterly stunned. 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.
"To discover 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 expects the 53-year-old music mogul to be a changed man when he becomes a father for the first time, and admits Simon's impending parenthood has already given him a "new lease of life".
She added to HELLO! magazine: "I can see he's excited, nervous, all the things you experience as a new parent-to-be, but that I just never expected to see him experience.
"He's calmer, more philosophical about things. I've noticed a special spark about him - like he's got a new lease of life.
"He has even started doing things like going for walks. It's sweet, he never did stuff like that before."