Hi. Greetings. Sun tanning can occur even with short exposures and this is a protective mechanism of skin which is called immediate pigment darkening. Don't worry, continue to use a sunscreen of high SPF around 30 - and reapply every three or four hours if exposed to sunlight for long duration. See a dermatologist near you for any further assistance. Also make sure you are having a good breakfast and lunch daily and are well hydrated when you are outdoors. Hope this helps. Warm regards.