Please diagnose my tropical island skin sore

About a week ago, I developed an unusual sore. I noticed it when I accidentally brushed my hand against the side of my shin. A crusty weirdly-fragile scab came off. And an oozy sore was there. As the day progressed, the area around it became redder and slighty puffy. It would not form a scab. I started putting Neosporin on it right away. There are no dermatologists on the island, so I'm hoping someone on Boing Boing can figure out what this is. It's currently about the size of a nickel, and the red blotch has been increasing slowly. The center has stopped oozing. It doesn't itch. Is it ringworm? Leprosy? Flesh eating disease?