Sounds very familiar to what I have been through with both of my boys. They went from happily walking into bed themselves to being very upset about it.
It could be a number of things. Seperation anxiety, teething (those darn molars could be making an early appearance) or it could be that she is overtired like you said.
Just keep doing what you are doing. If she is only upset for a few minutes then I wouldn't worry too much about it