It is called tinea versicolor. I have it as well. It is caused by too much yeast on your skin. You can get a prescription shampoo to wash your body with, but it kind of burns. I haven't had much luck with it. My dermatologist prescribed me Diflucan 100mg (same medicine for female yeast infections). I take one a day for 7-14 days and it makes it go away. It will always come back (1-2 times a year). It is always worse for people who live in tropical climates like I do here in Florida. I did do some research that said sometimes you can keep it at bay if you take a medicine like diflucan once a month. I haven't tried it yet. Just google tinea versicolor and you can find lots of info. Supposedly it is not contagious, but I never had it until I had a long time relationship with a guy who had it badly. Also, my roommate that would wear my clothes sometimes got it as well. But, neither of my daughters have it nor does my husband of 8 years now. I have suffered from it for 12+ years. Good luck and please don't hesitate to ask me any questions you may have.
I also have these white spots on my back...it a type of fungus. I have used a foot fiungus cream 2 times a day on my spots for about 2-4 weeks and they go away. But they will come back if not in the same spot then in a different one. Sweating and heat can bring them out also. When you start to see the lightness in your skin begin with the cream at that point....normally it will keep it form getting any whiter. You can get the foot fungus cream at Walmart. Also my derma said you can use Selson Blue shampoo. They will make the spots crust over and be gone. My mother has these spots also and used the shampoo and it worked for her. I havent used it yet because I only have 4 small spots on my upper back. My mother on the other hand left it untreated for about 20 yrs so it was deep rooted and she had it all over her torso. You can get it from lots of things. She got it from the beach when she went to Florida. Who knows where I got it. But good luck!!! Its not the end of the world.....a pain in the butt yes.