Since you're not on any medications that could possibly cause a life threating rash, I don't think you have a thing to worry about. The pic you took looks like it could be a sore from maybe a mosquito bite or something else, but only a doctor can tell for sure. Either way, it is extremely rare for anyone to develope a life threating rash without a causitive agent.
It sounds like you would do well to see a psychiatrist, to get to the bottom of your fear of life threasting illnesses. You could have an obsessional problem that medication or cognitive therapy could really help you with.
Believe me, I've suffered most of my life with depression, panic disorder, OCD, etc. and seeing a psychiatrist along with medication has helped me out greatly. Don't be afraid to see a doctor and explain the thoughts and feelings you have, they are there to help you.
Keep your chin up, there is help for you. You are not dying!!!!!!