Rashes can be from lots of things. Have you been in the sun lots in the last couple of days? Have you used a new soap or laundry detergent? Have you had this sort of rash before? Benadryl will work on your rash if it is from an allergy. If it is a rash related to an underlying autoimmune disease then Benadryl will not help (except if its from an allergy to the sun). Your doctor will be able to help you more with this. What I would do is photograph your rash so that you can show it to your doctor. Do you have a digital camera? If not a normal camera will do. I imagine it will still be visible for Monday's appt, but still it will be a good thing to have pictures for the future. Hope this helps!