Why Irrational fear just developed/realized?

I am having a bit of trouble coming to terms with a fear I have just come to realize I have. It is bizarre to say the least. I know lots of people with irrational fears of clowns or little people or heights and I always thought to myself " I guess I'm lucky that I don't have any of those issues" but then tonight happened and I'm wondering what the heck is up with me.

So I walked around the corner into my kitchen and one of my dogs was standing in the middle of the floor on her hind legs walking around. even just typing about it makes me freak out. I froze up. my heart started beating and then she kind of turned around while still walking upright and looked at me and I really flipped.

I ran to my husband and tried to tell him about it but I was laughing too hard- I am a nervous laughter person -let me stress I did NOT find the situation funny I WAS TERRIFIED. I was tearing up and trying to describe what the dog was doing and my husband was trying to get me to calm down. Finally I got it out and he thought it was funny- understandably. But I was really freaking out, and I know how stupid that sounds. But for some reason seeing my dog walk around like a person is really scary to me. I thought maybe it was just my dog and the fact that I was surprised when I came around the corner but it turns out that when I searched for "dogs walking like people" videos I couldn't even watch them without panicking again. I have seen little dogs do it in person and never had an issue.

I've been trying to figure out why I'd have this sort of response like maybe big dogs just look so human and the idea of the acting like humans is scary to me? I don't know. All I know is anytime i think about what I saw my heart starts beating really fast and I feel like I'm going to have a heart attack. I tried recounting the situation later with my husband and I started the nervous laugh thing again and had a really hard time calming down.

First of all does anyone else have an issue with dogs walking like people or have you heard of people being terrified of that before?

If so Why do you think someone like me who has had big dogs their entire life and never been afraid of them suddenly be afraid when seeing one walking around like that?

I read if you have an irrational fear that you should confront it but I don't know anyone with a big dog trained to walk on its hind legs, my dog is not trained to do it, obviously had no idea she could or did walk around the house like that until today. Dogs jumping up doesn't bother me.

(This is not a joke I was terrified and I now feel horrible that I used to pretend to be a midget to scare a friend of mine because I honestly had no idea how horrified she felt until now.)