Invited a co-worker to a birthday party

A few of my co-workers are having a birthday party for one of the secretaries at work. I asked another secretary if she wanted to come. When I did she did not respond in any way, she sat there at her desk looking at me. I started feeling uncomfortable and I told her if she wanted to come, let me know. I then walked away.

In the past, if I invited someone to a social event people would either tell me they were coming or say they had already made plans and they couldn't come. I've never had anyone to sit and stare at me until I left.

I can't read her mind but do you think she simply did not want to attend and couldn't think of a good excuse of why she couldn't come?