My cat has two small clumps of fur on her neck that's very close to her skin. She's short haired and brush her regularly plus she grooms herself multiple times a day. The clumps are so close to her skin that it's hard to tell that they're there unless you're petting her and you find them. Because of that, I'm not sure if its just matted fur.
~update~ The mats are actually a sore, I found that out when a scab fell off of it. She's also become light sensitive in the past couple of hours, her eyes being a little red. I should also add that last week on Thursday, she swallowes a 4 inch piece of yarn that she has not yet passed.