Is OTHER wrong in the post ? POST : Just quickly about me: 15 years old, boy, dumb af, have been dating my girlfriend for 2 months now and she's great. I am 100% confident I won't ever cheat on her.

However, I don't know why but if I see a friend that's a girl and she's dressed fancy or something  I'll say things like "You look beautiful today," just to complement her. Now, there's one point you can judge me on, but that's not the main part.

Today, after my taekwondo class, I was waiting outside to get picked up. Friends pass by so I say "bye," and such, and then this girl (I don't even know her name but I interact with her every now and then at the dojo) passes by, I say "bye" as usual, and she responds with a "bye," but she goes back to the building after going to her car so I ask "did you forget something?" and she said "No, I'm going to change clothes on the bathroom." I just say "oh, ok" not thinking much of it. 

Now here's the important part. I must say, she is beautiful even in her training clothes. But when she came back out of the building, I saw her and she stunned me how much more beautiful she looked in proper stylish clothes and a "WOW" just came out of me and I couldn't contol it. I explained saying "Sorry, it just surprised me how beautiful you look" and she awkwardly replied "Oh, it's fine. Thank you." 

I felt bad after that... my girlfriend doesn't know and I'm not sure if she should know, but I just feel awful.

So, would I be the asshole if I didn't tell my gitlfriend what happened? Or would I be the asshole if I did tell her as she would be really upset? Or maybe I'm just an asshole for saying "wow, you look beautiful" to another girl... Maybe I wouldn't be an asshole if I tell my girlfriend honestly? But that will probably do more harm.. Now as I write this I really feel like an asshole 

Tl;dr - I usually see this one girl in her training clothes but today I saw her in proper stylish clothes and a "WOW" came out of my mouth, even though I have a girlfriend who I would never cheat on. Am I the asshole?