Look at it from your PI's perspective - she's gotta deal with your colleague. She doesn't have to deal with you. Also, it's research that you handed off - not research that are continuing to work on. If one person can go, it should probably be the person who has the most recent exposure to the subject. Yeah, you can be cranky about it but it's not like the decision is surprising. Look at the bright side - my mother's PI took her thesis and presented it as his own. Got grants that kept his lab going for a year because of it. My mom couldn't do jack shit about it because she'd been committed with bipolar disorder. He faced no consequences whatsoever and my mother fled to the community colleges.
I am more pissed about the fact that I was not asked if I wanted to go to the conference... I wouldn't have a problem with my colleague going, not at all. But now I see an abstract, with my name on second place, with my data (and my data only, I started the project) presented at a major conference in my field by somebody who had nothing to do with the project a month ago :(