We have found out it's not acne, but allergy, we don't know what it is…

We have found out it's not acne, but allergy, we don't know what it is for. The doctor put me on a diet for a week (to give up cow's milk and sweets). And prescribed Lacari cream. On the cheeks everything started to go away, but on the chest a rash appeared. What could it be from? How to check infectious or allergic rash?