What could be causing white spots on my back, neck and face?

Perhaps fungus: It is impossible to tell from the description. A fungus called tinea versicolor can cause white spots which become more noticeable after sun exposure. Another condition which cause white spots is vitiligo where there is a factor causing the pigment cells to stop making pigment. A dermatologist will be able to tell the difference and properly treat the discoloration.

Possibly fungus: You may have tinea versicolor. Apply selsun blue shampoo to skin, leave on for a few minutes, then rinse off in shower. Do this 2-3 times per week for a month to help treat the fungus. The color will take time to get back to normal, but the treatment will prevent the condition from getting worse.