I can empathize with you on this. Having anxiety situation largely around this since a young age it has been essential to develop personal methods to overcome this anxiety. Personally i found occupying my mind at times when I was free to think most was helpful, listening to music or an audio-book while falling asleep (when the anxiety was at its worst) would keep my mind distracted. If the anxiety is seriously affecting you consider consulting a medical professional, even a check up helped me put my mind at ease.
It depends if you are really feeling something physically or emotionally. You can stop yourself from thinking too much by having a check up and to tell the doctor the situation as honest as you can. That way the doctor could see if you really should be worried or not.
Most of the time we get stuck up about how sick we are, how wrong things come out, and even the slightest sign of sickness brings us down, the best medicine for this is to do somehting you enjoy, talk to your best friend, read a book, go out for a evening walk with your pet if you have one, the best way to stop thinking about whether you're sick or not is to think about something else! The rest will come out by itself, everything is in the mind.
One thing you can do is to get yourself checked professionally. This would most likely set your mind to rest a little bit. Short of that you can also ask yourself why you think you are sick. Based off what evidence do you think you are ill? Now look at if it is a reasonable assumption that you really are sick. Sometimes you need to take another, more scientific look at things to quiet your worries.