Well, I'd say that in this instance you should trust your boyfriend. He may be lying (or maybe he isn't) but why would you take the word of his ex over his? You asked and he said it didn't happen.
She is clearly somebody with a vested interest, whether its pure hatred or just total assholery. The point is she's a biased individual here who might still want him. Now, I'd make sure you let him know that he is not not deal with her at all: no texting, calling, visiting, seeing, hanging out, etc. He should understand and respect this given that this chick is clearly trying to break up your relationship.
So, at the end of the day, fool you once, shame on them...you just have to believe him. But if something happens that gives you any suspicion..well, then dump him with the quickness!
Has your man done anything, even slightly, to make you think he might be cheating? If he hasn't, then there's no reason to take an ex's word over his.
We all know, exes aren't exactly known for being objective about their former partners, and if an ex wants someone back, they are highly motivated to spread lies and sow seeds of doubt in the current partner. Not saying blindly take your boyfriend's word, simply because I have no idea if he did or didn't, but his word should carry more weight, unless his own actions have done anything to suggest otherwise.
PJs right...relationships are nothing if they don't have trust.
Only YOU know if this weirdness is a pattern of her wedging herself into your relationship, or a single "Hail Mary" to see if you will leave him so she can have her boyfriend back for Christmas.
I hate being jaded and suspicious...but it's often human nature for those of us who have been duped/manipulated, etc. Usually, I start off being the ultra trusting GF, who then gives one too many chances after red flags, so, with that warning, here is my two cents:
When you're casually hanging out, and you know he has his cell phone, bring up some interesting anecdote about the newest cell phone app, or some issue with your phone and then (as if a random thought) ask to see his phone for a sec. He should hand it over without hesitation or fearful look in his eye, and without making you wait a few minutes while he hides the screen and messes with the contents before giving it to you.
If you asked a friend to see his or her phone, they would respond without hesitation. The only reason your BF would hesitate is if there is something to hide or he is concerned about what you might see. For those who might say it's "entrapment" and you're looking for trouble, no, you're not. Sharing should not be an issue if in an honest, committed relationship.
It works.
Why is she coming for Christmas. That makes no sense?! I would uninvited her cause that's a major problem.
Seriously, if she is simply in the area for Christmas then just avoid her. Both of you.
I am not the jealous or suspicious type but if someone wad trying to break up my relationship with a vengeance I wouldn't be cool with them being around.
Trust your boyfriend. The ex is just jealous and trying to ruin your relationship... and besides.. I'd have that wild fantasies of your in all the rooms.
screw her.. Ignore her at all cause. But you know believe in yourself and your boyfriend. screw the damn ex. Let the past be the past what she loss is your gain.