It's not that you escalated too quickly. I think she was probably surprised that you immediately assumed she was sick, though. I would have at least asked what was wrong before doing that. She could have just gotten an F on a test or something. That's not what's important though. You made an approach and had seemed to have had full confidence. That's great.
Most people in general are pretty shy. She was probably just nervous because an attractive (hopefully, right?

) guy approached her. You should have grabbed her number when she got up because I think you had it in the bag.