First off: :crackup:
Why am I laughing? Because you're allowing her to think you did something wrong - which you didn't - instead of turning this into an opportunity to make her your girlfriend.
You say you're not official with this girl, right? So, technically, that means you are still entitled to talk with anyone you like, or date other people. If she gets upset with you that you're still talking to them, it's her way of saying "I thought we were exclusive" or it means she feels as though you two should be exclusive. If you don't feel the same way and aren't ready to have her as your girlfriend then it doesn't matter either way...
BUT, if you are looking to make her your girlfriend, the secret is to use this incident to get what you want. Here's an example of how I might have handled it:
HER: I found out you were talking to those girls you used to talk to!
ME: Yeah, and?
HER: How could you do this! I feel betrayed?
ME: Hmm, interesting, and why would that be?
HER: Well... because... I thought you were just seeing me, and not still chasing after other girls!
ME: Well... no offense, but as far as I know we've just been dating. We haven't made anything official yet, and as far as I know I figured you were probably still talking to other people as well.
HER: What would make you think that?!?
ME: I dunno, we never had a conversation about making this thing official, so I figured maybe you weren't trying to do that sort of thing yet (see, saying this now puts this whole thing on HER and off of YOU)
HER: Well, I still don't like that you're talking to them.
ME: I hear what you're saying... so, what does this mean? Are you saying you're trying to make this thing exclusive?
HER: I mean, I guess I thought we already were... but you're right, we haven't really talked about it.
ME: Are you saying you want to have that talk now?
At that point, you guys get to talking, negotiate terms of what her being your girlfriend would mean, then tell her you're cool with trying it out and seeing how it works. But the point is, you've now turned an incident where she was being all mad at you, into a discussion about it being a misunderstanding of where you two were at and where you both want it to be.
The moral of this story is, never let how a woman reacts to something take control of the situation. Always remain the calm, rational one who isn't affected by her emotional outburst, and understand that she is only going to react the way you ALLOW her to react. Hope this helps - oh, and for more killer tips like this click on the link in my signature to check out my daily podcast!