Is it normal to change so fast

I'm a normal straight guy or at least I was. I'm in my mid 20's graduated from a good college and have a good job. I've had a few girls when I was in school and am in a pretty good relationship now.
Last weekend we went horseback riding and I fell off and hit my head. My girl said she thinks I was unconscious for a few minutes but I don't remember it at all.
My problem is that when I woke up something had changed. We rode back to the barn and on the ride this girl I used to think was so sexy seemed so plain and uninteresting to me.
When we got to the barn the groom a Latin guy seemed the most sexy person I had ever seen.
I haven't had sex with my girl since the fall and she no longer appeals to me at all but I find certain guys so attractive I almost get erections. I have never even thought of sex with a male before but now its all I think about.
Is it possible to become gay after being unconscious from hitting your head? Is it normal?