How am I supposed to feel about my dad's girlfriend? Is it my fault if I immediately dislike her?

So, my parents have been divorced since before I can remember, and my dad hasn’t dated anyone since I was maybe 6 (or if he did, he didn’t mention it), and now I’m an adult. He told me recently that he’s dating a woman almost 20 years his junior (meaning I can count on one hand how many years older she is than my own SO) and her name is “Dixie”... He told me on Father’s Day, and then she facebook friended me on Tuesday (even though WE HAVE NEVER MET!) Before my dad told me about her, he had mentioned several times that he was going to a specific concert, and I had even asked him if he was going with anyone, and he said no. And then suddenly, weeks after the event, he tells me actually it was with her, and they’re dating. It was a “date-date”, not their first “date-date”.

I feel like I’ve been deceived, and that there are probably a lot of other things in his life he’s not telling me.

I didn’t accept or reject her friend request. It’s still just sitting there…I don’t know what to do about that…I’m just leaving her in friend purgatory for now…

I don’t know, I don’t like the whole situation, and I don’t like her. [Edited to add]: To expand on that, the few things I do know about her, I don’t like, and I don’t know anything about her that I like…and that makes me so uncomfortable. I just walk around feeling like I have something I need to get off of my chest.

I just wish he had been straightforward with me in the first place…