Hello, Welcome to Just Answer and thanks for your question. You are right. You are a hypochondriac. Seborrheic dermatitis is very, very common....about as common as dirt, and nothing to be concerned about at all. You may recognize it by different names (commonly it refers to dandruff in adults, cradle cap in infants). HIV is a risk factor as it is a risk factor for everything else because it lowers one's immune response. However, the vast majority of people have no risk factors but get it anyway. If you are truly concerned about your HIV status you should get tested.
Have you thought about seeing a psychiatrist for your anxiety disorder? Treatment would put an end to all the irrational fears and needless worrying.