Did you split up last year, I only ask because (particularly if you have DCs) Christmas and NYE are such landmarks, you may have cried in private and put a brave face on, or you may have felt strong and purposeful and past all the grief and worry.
Either way if there was ever a time you were going see how you felt with ex, "I am so over you, I am immune to any stunts you pull, I can sleep with you and not give it another thought" having survived Christmas, perhaps mid-January was the most likely time to test this.
If no thrird party was involved back then, are you both still free agents?