Red Rash On Stomach, Chest, Underarms. Suspecting Syphilis. No Other Symptoms. Due To Any Underlying Condition?

I have this red nonitchy rash on my belly and chest and underside of my arms. It looks like measles , though I have had measels before They dont hurt and are getting less visible. My doctor looked at them and said it was possible that I contacted std , syphillis. I have had no sores of any types, no painful urination , no marks on my feet or palms on my hands like he said often happen with this std. I get the results on Monday. if it is that, well I will deal with it. Is it possible that a rash like that can be caused by something else, like a reaction to something else