First, I'd like to be clear that I agree with your opinion that cheating should not be tolerated; it is unfair to other students, and generally has a negative effect on both learning and motivation in coursework.
I think honestly informing your instructor about observations or experiences in an exam is generally helpful, and we are almost always greatful for the feedback. In general this should not be something you can "get in trouble" with your instructors over.
That said, I'd like to point out that you probably don't have any responsibility (or authority) to make judgements about your classmate, or the gravity of her offence. I know that when you are stressed by time-constraints and a challenging course load it is tempting react strongly to the thought of someone cheating. It appears that your classmate was doing something wrong, but the fact is that you don't know that for sure. If you think about it in that light, you might appreciate that you are relieved of this burden (judging her and deciding her consequences). Hopefully, your professor can look at the incident more objectively and take appropriate action and/or appropriate precautions for the next exam.
Your reaction, accurately reporting what you saw, was the right one. Now you should let the instructor deal with it, and avoid jumping to any undue conclusions.