The pair were both invited to the lavish bash in London, but it has been claimed by a source that Myleene confronted pregnant Frankie after she "blanked her" soon after arriving.
Graham used to work as a bodyguard for Frankie's band The Saturdays and Myleene was said to have previously invited the singer to her house on several occasions.
The source told The Mirror: "Myleene smiled at Frankie but was ignored. The poor thing felt really stupid and angry. She stormed over to Frankie to have it out, asking her why she kept being so rude."
Frankie is said to have mumbled that it was "awkward" before Myleene headed off to be comforted by Fearne Cotton and Claudia Winkleman.
Myleene's agent has refused to comment on the incident, while Frankie's rep also remained tight-lipped.
Former Hear'Say singer Myleene first began dating Graham in 2001 after he was employed as the band's bodyguard. They had two children together, Ava and Hero, and married in an intimate ceremony in 2011.
But just six months later, Graham left Myleene on her 34th birthday. The couple had a 'quickie divorce' and it was finalised in April.
Soon after their split, Myleene thanked fans for their support. Taking to Twitter, she said: "Thank you all for the thoughts and messages of support for me and my girls. They've given me comfort, courage and hope #strongmamas."
She has since said: "At the beginning, every morning when I woke up, I thought I had taken a bullet to my heart.
"There were days when I didn't want to get out of bed and my mum would say to me, 'It's OK, you are just grieving.' And she was right – it is a kind of grief, a grief that you have to work through."
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