It could be Bier's Spots, have a look on google and see if that sounds like you. Xx
Hi,
Sun or UV exposure may cause pigment changes such as freckles and white spots especially on the legs and backs of the hands and arms. This occurs due to destruction of melanocytes . Other differentials include tinea versicolor or even vitiligo, depending on the extent of depigmentation. At your age, cancer is usually not included in the differential. If it persists, have this evaluated further for proper management.
Take care and regards.
omg i am only 13 and i have the same problem its on my arms and i can see the white dots all over my arm clearly in the SUN i have no i dea wat this is but i am as worried as u r that it might be cancer hopfully not. cuz im too young.