What is this thing? Is it ringworm? Or what is this skin problem?

It could indeed be ringworm.  You should definitely take her to a vet and have the vet check it with a Woods lamp (like a black light, most ringworm will glow green under it in a dark room).  A vet can also do a fungal culture.  The sooner she gets on an antifungal, the better, but I would not put any antifungal on it until you have seen the vet.

It certainly looks like ringworm! But, the only way to be sure is to see the vet. Like Red Top Rescue said, sooner the better!