This could potentially be ringworm, but only a vet will be able to determine what’s going on. Skin issues are very difficult to speculate about online.
While it certainly could be ringworm (and we're seeing a lot of that on here right now), it's an awfully large patch not to have any crustiness or raised bumps at all. I'd have a vet look at it to make a definitive diagnosis. Something that big is unlikely to clear up on its own.