Hmmm well if you guys are work friends, then she probably felt akward saying no. Meaning, maybe she was just trying to be nice.
You guys work together, so you do have some kind of relationship, and she knows she will bump into you, everyday. So she probably felt, obliged to give it, to keep a normal work relationship, with no akwardness. You know what I'm saying?
But, regardless of the real reason she gave it to you. I think the only way you can answer your own question is by calling her, and seeing what's up.
If she likes you, she'll talk to you alot, on the phone, and maybe agree to hang out with you.
If she blows you off alot, or hardly answers your calls, then it means she WAS just being nice.
So, since you have it (her number), take advantage of the opportunity:) You are one of the lucky ones she gave it to.