What could cause white patches/spots on my chest, stomach and back? (Photo)

Attached is a picture of my back.I have white patches/spots on my chest, stomach and back. They are more visible the more tan I get. It started in January and it is spreading. i use indoor tanning beds 2x per month. I am on no medication except St. John's wort, which can cause sun sensitivity. I'm wondering if is due to a fungus, or the sun.

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