Why am I irrationally afraid of amputation?

I consider myself a pretty rational thinker, and I know how uncommon amputation is. I do not participate in any unusual activities that would put me at risk for needing to have a limb amputated. However, I am really, really afraid of amputation—to the point that it almost interferes with my daily life. I spent at least a half hour every day thinking about how afraid I am of having a limb amputated at some point in my life because of some freak accident. Then I become afraid that, since I am so specifically afraid of amputation, I am putting myself more at risk because of the irony that would be associated with having amputation happen to the person who is most afraid of it. Why do I fear amputation so much?