it sounds to me you suffer from ocd meaning obsesive compolsive disorder?. i use to suffer with ocd but mine was cleaning and checking things over and over again everybody has differn't types. mine started when my beloved brother was killed back in 2000. anything can trigger it but normaly something bad thats happend the way i got my self back to normal was say to my self im just being silly and force my self not to be so compulsive. it takes time you can get medical help for this and it could also be your a hypercondract?. and thats a medical problem to there is help 4 this im surprised your gp has not picked up on this???. and no i don't think your gonna get a blood clot your young something must of triggerd all these fears and feelings you keep getting it sad... you should have no fear the way i personly see it you could get run over by a bus or something see what im saying.. enjoy being young and speak to someone ie parents brothers or sisters friends or perfesionals. or of course theres always us thanks lynne from the uk..
You sound just fine and need to accept this. I know it's easier said than done, but therapy can help you with this. Our anxiety can make us becomed fixated on a certain health issue which consumes us, when there is no reason for it. You have no reason to have this fear so therapy would greatly benefit you in getting over your fear. I want to see you living your life and not worrying it away. Stop all the testing and seek help with a psychiatrist. I suggest a psychiaitrist because should you need medication, they are the most knowlegable, and they can refer you to the proper therapist. I know it's tough, but hang in there, you're going to be okay. We're always here for you!