Because you have an anxiety disorder. Your thought processes (and the rest of ours!) are wired differently than the average person, who would in fact be relieved that something was a false alarm. And a little more specifically, reassurance-seeking is one of those coping mechanisms that "relieves" our acute anxiety but only tends to strengthen the idea that there IS something "wrong" and in that sense we were right in the first place!
(GAD here, not health anxiety, but the reassurance seeking rings true for me too)
So what do we need to do with that? Retrain our thoughts....with CBT, mindfulness, meditation, etc.