I have found that sometimes being nice to others is seen by some as being weak, a reason to be taken advantage of. A lot of times us nice know what is going on and we leave feeling like suckers, but we want the others to be big enough to rise to our niceness and reciprocate. Sorry, that isn’t going to happen. Especially in our world of eat or be eaten. Its sad the way our world is, the nice people are always the ones to suffer most while it is those who are the most cutthroat and maybe even evil, at least mean are the ones who prosper (in most cases). Why should nice people suffer for being nice? They shouldn’t.

Now while I don’t know your coworker, almost always in these types of situations there is a motive. She is trying to undermine you for a reason, is it to feed her ego? Get herself higher in the company? Or move you out? if that is the case then it really is beneficial for YOU to take charge of the situation in a way that not only makes you look professional, but also don’t let it turn into chaos because she is probably counting the times she has “Successfully” given you help and she will may use your handling of the situation against you.
In the book “Opening the door of your heart” Ajahn Brahm tells a story about a snake who has a bad reputation among the people for being mean, he had no problem biting. As he got older though he wanted to settle down so went to the local temple to learn the ways of Buddhism. He gave up his mean ways and just meditated, he didn’t even hiss anymore. People would walk by with out a reaction from the snake and they thought he was dead or dying. Eventually some kids started throwing rocks at him and he ended up all bruised so he went up to the temple and told the buddhist monk off. The monk saw him coming, [“what happened to you?” The snake exclaimed back, [“Its your fault I knew this meditation, Buddhist stuff was garbage”] The monk shook his head, [I told you not to bite, I didn’t tell you to stop hissing.”] The moral of the story is mean people will beat you up (not always physically, but maybe) if you don’t at least act like you will bite.
Hope I could help you