I have had a few different rashes on my Caminos. I recommend that you stop at a pharmacy and ask the pharmacist for some help. Sometimes I have had to stop twice before I got the right treatment, but I have been so impressed with the pharmacists' willingness to diagnose and treat things such as rashes. Twice I had something they described as a sun "allergy" -- not sure exactly what that is, but the cream they gave me worked. (That rash showed up only on the parts of my skin exposed to sun.) Another time I had an infection that needed to be treated with an antibiotic. I have relied on pharmacists in Spain at least five times. Note that, on occasion (once for me), the pharmacist recommends going to a doctor or ER. My advice is to follow that advice, too.
If you don't speak Spanish, you might look for a pharmacist who speaks English, or ask another pilgrim who speaks English and Spanish to go with you. But even if you just walk in and point to the rash, you are likely to be directed toward the right treatment. At least that has been my experience.
Best of luck.