I think I can explain this one.... it's related to change, either good or bad. When you experience a change in your life it causes anxiety because you are going into an unfamiliar or uncertain territory.
Change can cause anxiety because we are out of our comfort zone.
Janet maybe your anxiety was related to the possibility of a loss of income and the impact that may have on you, even if at the moment that doesn't apply. Having the security of that job, even though you hated it, was a safety net. It may be something else, I don't know, but even though we can often look forward to change, it is still such an unkown that we feel anxious.
Hope that helps a bit.