Texting between iMessage and Blackberry

I am an iPhone user. A couple of weeks ago, I received a Blackberry for work. My job provided it as well as the phone number. When I was setting up the Blackberry, I sent my girlfriend a text. She was sitting next to me. The text went through to her iPhone as a text. She responded to the text and it sent as an iMessage. Obviously I did not recieve it, but someone else responded! I do not understand how that happend. Here is what it looked like.

Text Message

Hi <~~ me, text box in white

iMessage

Hello <~~~ girlfriend, text box in blue. I did not recieve.

? <~~~ sent back in white box, still under iMessage. From someone unknown.

Then tonight, I received a text from someone that I don't know. The text came through as if it was in the middle of a conversation. I don't know what the deal is, but I am kind of worried. Like I said it is my work phone and I don't want someone with an iPhone I work with try and text me and have it go to another iPhone user instead of my Blackberry.

The service provider is At&t if it matters.

Please Help!

p.s. I am not a Blackberry fan. I would much rather use my iPhone. But it's what my work provides.