The singer wrote the song after tabloid reports that Fielder-Civil got 31-year-old Gilleen Morris pregnant when the pair met at a rehab clinic in Sheffield, The Sun reports.
A source said: "Amy still felt they had something between them and that one day they'd be together again. But this news destroyed her.
"She sat down with her guitar and started writing 'The Ultimate Betrayal'. She keeps repeating the line: 'Blake a baby, no, no, no.' It's very haunting."
The source added: "Amy's best material was written when heartbroken by Blake, when he was two-timing her.
"But with all her problems we would rather she was happy than writing brilliant, emotive songs."
Winehouse is currently in St Lucia with producer Salaam Remi writing and recording tracks for her upcoming third album.