I used to have a similar experience, and still think I'm overreacting when I'm ill. In my case it comes from not having been taken seriously as a child (my parents stopped taking me for check-ups when I was 9, even though I had several health issues). I am now dealing with unexplained symptoms (as of yet), and constantly tell myself they're not real. However, they won't go away if they are "all in your head". Please do take yourself seirously.