I know that when a girl says this I've already kinda screwed up, but I was wondering how to reply to this?

In what context does she say it?
That is,..What happened in your conversation that led her to say your shy?

he was probably acting like an afc.

Perhaps..
"You seem shy" can have heavy sexual connotations.
I'm getting a feeling that there was a display of weakens during her escalating arousal. I feel that recovery may be possible. However, more info is needed to "debug" this failure.

Yes, sorry more info... I'm working at a bar and I'm new to it. It happened that she asked for a drink I didn't know how to make properly + didn't know the price of. I also didn't quite understand her (English wasn't her first language) these things led me to to be quite unsure of myself (a lack of confidence in what I was doing was certainly visible).

There wasn't much conversation (just about the drinks and a couple of pleasantries), but I was getting many IOIs. I am new to pick up, but I came across as more AFC than I typically am because of the circumstances.

(thenightfox... I'm not really a fan of the third person).

"I'm working at a bar"
"There wasn't much conversation"
"I was getting many IOIs"

You were getting played,..she wanted a free drink.

You didn't give her the drink right?

Lol no she paid for the drinks... It didn't cross my mind that she might be trying to get free drinks.

Guess I'll just handle things better next time and be more at ease about not being experienced in my job.

First off, bartenders naturally are going to get the most attention at a bar because the dynamics of the social environment.

Bar comes from Barrier. And bartenders are the people who control who gets a drink first. They have so much unconscious power is ridiculous.

Therefore, when your working just have fun, you already have the power.

When the night is over and she closes her tab write..

your number_________.

Or if your allowed to, let her hang out the bar after its closed. It makes the person feel AWESOME... I was talking to a girl bartender and she let me stay after everyone else was gone and you feel like a VIP.