There is a walk-in doctor’s surgery just around the corner from / attached to the chemist. (It appears some pharmacies have this while others don’t.) The prices are clearly displayed - with a consultation costing 50 pesos. When I arrived someone else was in there, but I waited 2 minutes and then was seen. Dr Dominguez was very kind and immediately understood the situation from my bad Spanish. After examining and listening to my stomach, he prescribed antibiotics, loperamide and probiotics, which I got from the pharmacy for 194 pesos. In total it took about 10 minutes and cost around £10. Don’t suffer in pain and let a stomach infection ruin your holiday!
Eye in Tulum there are only 3 medical offices, and this is the only cheap I know. I highly recommend coming at a time when children are in class because they take a long time to consult. The attending doctors graduated from good universities so there should be no problem for the service.