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

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.
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.