Well, the good news is that in all probability it is not a viral infection as they are generally accompanied by severe discomfort and/or itching. You mentioned that you are worried that it may be a blood disorder. If you have been anaemic in the past or if you still are, then there is an increased risk of you having a blood disorder. There is no harm in going in for a blood test, as a test whether positive or negative will bring you closer to finding out the cause of your rash. Since your rash disappears periodically, it probably isn't a fungal or bacterial infection either. This would therefore leave you with very few possibilities.
I would think that these might be hives. Although hives are generally itchy and painful, it is not necessary. This would account for its disappearance and revival. Unfortunately there is no cure for an allergy, but the symptoms can be treated. You will have to try to identify the common grounds between several attacks before you can pinpoint the cause of your reaction. Once you have figured out your "trigger", you can avoid it.
answered by G M