Kourtney Kardashian should've been thrilled after finding out she was pregnant with baby No. 4 — only the reality star, 37, is living her worst nightmare, and it's because she doesn't know if the father is long-time love Scott Disick or new boy toy Younes Bendjima.

In Touch can exclusively reveal how the mother-of-three found out about her little bun in the oven. "When she told pals a few weeks ago that she missed her period and fears she’s pregnant — she also said she didn’t know who the father is," a source revealed, adding that she had reconciled with Scott, 33, while she was still hot and heavy with Younes, 23.

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"Scott thought a reconciliation was in the cards for them, they had discussed having another baby," the source said of Kourtney and Scott, adding that during their Mexico vacation they failed to use a contraceptive. "Kourtney is very fertile. She has a big problem."

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Around the time of her reconciliation with Scott, she was also spotted with the boxer-turned-model on numerous occasions. "Kourtney has a thing for boy toys, and Younes is just her type,” the source continued. "She’s been boasting to pals about his amazing six-pack, telling them she can’t keep her hands off his sexy body. Scott hit the roof when he heard she was hooking up with him.”

As In Touch previously reported, Kourtney was also rumored to have hooked up with singer Justin Bieber, 22, during her break with Scott.

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The brunette beauty broke it off with Scott in July of last year, after photos surfaced of him getting cozy with an ex-girlfriend in Monte Carlo. The couple currently share children Mason, 7, Penelope, 4, and Reign, 2.

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Scroll through the slideshow to see Kourtney and Scott's cutest moments!