In the corporate world it is very common to have people making big deals out of nothing. If this is the Public Sector, even worse.
If she's a visible minority, female and age over forty, HR will side with her. They don't even want to hear what you have to say, she won the battle.
If I were you, I would return the pen and never ever speak with her again, unless there were plenty of witnesses and strictly work related.
It is very dangerous these days, watch your back