The fear is scary and can be controlling, I am sorry to hear about your worries. Have you seen a GP ? they can often rule these things out fairly quickly. I am also very slim, a little older in my early 30's. I went through a phase earlier in the year where I wound myself up to the point I was certain I had an issue with my left calf, I wont go into details as that can bad to read, needless to say it was fine.
Very often anything like that comes with swelling which is particularly noticeable in slim people like ourselves, doctors also have a number of very simple tools at hand such as a D dimer blood test, which can help to put your mind at rest