Skin wont change colour.

While working on some decking during the summer shirtless I got sunburned on my back. I turned red everywhere except for a large circle on my lower back and a patch that goes from shoulder to shoulder about 10cm in width. These patches don't turn red after a shower either. On further inspection I have fount a few small patches on both my arms and legs about 1cm in size that also don't turn red.  

All these patches aren't raised nor are they dry or scaly and seem just to be a loss of pigmentation and not something on the skin.

I'm hoping it's not vitiligo, but for what i heard that would have burned before the rest of my skin, and would still turn red in the shower.

Does anyone know what this could be?