When Nathan overheard her say Daniel couldn’t find out she had slept with "him," why didn’t she just say it was Philip before she met Daniel? I’m sure Nathan would have bought that.
Right. First she slept with Phillip while married to Brady, then she has an affair with Daniel while married to Lucas, now he hears a secret hinting that she slept with someone while with Daniel. That would give anyone the impression she’s a sex addict.