Worried I am going to get fired from my job when there is no cause!?

I am having really bad anxiety over my job I am afraid I am going to get fired for no reason. I have no idea why. I am there when I am suppose to be I never call off. All my evaluations are great my last one which was almost a year ago was perfect. Under every category of my evaluation I got "exceeds expectations". But why am I feeling this way. My boss said I am the best auditor she ever had. Why do I have anxiety over NOTHING?? I'm not doing anything wrong.