imitation of someone I don't like? Help. Panic attack

(sorry about my english) --- So, there's this person in my class, and he's very annoying. He thinks only his opinions are valid and true, and things like that... for him, other people opinions are always wrong... It's very annoying to talk with someone like that. He's always saying like "Ok, prove me that your point is legit"..... about everything.... If I say that I like the color Blue, he will say: Explain me why blue is good..... Annoying!

That said, yesterday I was talking with a good friend of mine, and she asked me about which toothpaste I like to use.... i said X and she said she like Y. That's when things got terrible for me, because I naturally catch myself asking her to defend her choice for me... to explain me why Y.

Yes, the behavior that I completely Hate. And I was doing the same. And it was not an intrusive thought. No... it came naturally. That's why it's killing me. I don't want this! Now I feel i'm like this annoying guy. It's terrible.

Now i'm having panic attacks about it. Can someone give me a outside vision? Is it normal to end up imitating some behaviors?

I forgot to explain, I didn't get to the point of asking her to defend her choice, we were talking by text, and I deleted it just when I realized I was sounding like him! And there's another difference: When he asks everybody, he does it to prove they're wrong... But I was just trying to see if she could convince me that her toothpaste was good too. But anyway, the fact of sounding like him is what's disturbing me. The fact of his vocabulary popped up in my mind like it was mine!

I also started on taking Prozac 4 days ago. maybe i'm overthinking and feeling all of this because of that? Even before this fact happen, I was exaggerating the way this guy effects me... It's all my anxiety. It shouldnt be annoying that much. Other mates dont care about him being annoying that much as I do.

Right now it feels like the end of my world. Really. Like i'm not myself anymore. Like "how could this happen to me?" "it happens with other people"? I'm scared