Yep, definitely sunfading. I had a similar thing happen when I quickly had to move some ponies (unexpected visitors) I'd just bought a big lot and they hadn't been cleaned yet, and were laying out on my spare bed. I threw them into a bag, and they were in there longer than I expected, so some ended up with lines and marks from the fabric dye. I had to sunfade to get the marks off. I was heartbroken when I finally got them out, but thankfully they all survived, and yours should too!