Mid Bikini Bottoms

I have a lot of Sea Level bathing suits in many colors, this is the first one to fade - the first time I wore it!! Bought in May for my early July vacation (currently still on). What’s odd is that the matching top shows no discoloration. I do not wear sunblock, and I wore both pieces together in the same pool and same amount of time laying out. I did not sit in anything wet and the towel I used touched both the top and bottom equally. Not sure if I received a defective bottom? Will need to ask if Nordstrom can exchange, but not sure since they have been worn. Once!