That was not a very nice way to be treated. But you do not state there was any kind of contract or agreement on how whe was representing you. If you and she had an agreement (a Buyer's Agency Agreement would be typical), I think that might have dictated she could not take advantage of an opportunity that you would have liked to have. I know the proeprty you were looking into, though, and I am not sure it was such a good deal. The better way to do this going forward would be to find yourself a good buyer's agent and sign an agreement where you agree to work only with that agent for a set period (or until you are no longer satisfied) and that agent agrees to have a fiduciary duty to you.
I am an exclusive buyer's agent. That means I never have any listings and I only represent buyers. It is my belief that this type of representation is most helpful for a home buyer. I wish you the best of luck.