Irritation on hind leg

This evening I noticed that my cat has fur missing on the back of one hind leg. There is a red line of irritation there on her skin. I really want to find out what is irritating her. She always seems to be affected by things only on the rear end of her body, (underside or now, hind leg)... I know I should take her to a vet (during the holiday weekend, most are closed)... but I am just hoping perhaps these symptoms sound familiar to someone who might be able to give me suggestions of what is causing this. Thanks so much.

Could be flea irritation, she could've sat on something. My grandma just had something similar with her dog, and it turns out that the man who sprays her yard with insecticide didn't realize the dog could get out into the yard... little Angel went out, and went potty, apparently in the insecticide, and had a big irritation spot. Has there been any poison sprayed lately? Might wanna check. Best bet is the call the vet, if you can't get her to one.