You aren't her daughter. No matter how much you try to delude yourself by saying that, you AREN'T. And she doesn't view you as a daughter, she's said that.

It's ok that it stings to have seen her... but what would you have done if she'd warned you ahead of time? You would have either searched her out, spent the time dwelling and trying to figure out which girl is her real daughter, stayed home and gotten drunk and missed work. Or if you hadn't been working that day, you might have gone anyway to try to see her, or you would have been mad and thinking that she told you just to make you feel bad and to rub it in that she has a daughter and that it's not you.

Sounds like she went with the option of saying nothing as it was the only option that had any chance of sparing you having a meltdown.