Mystery Case

I have a mystery case! The two images above are of my arm. When I first noticed the lesion, it already looked like the first image. I couldn't recall anything that might have caused it, and I couldn't think of any trauma involving my arm. It appeared all of a sudden. It was not painful or pruritic. The area of central clearing was slightly raised. With strong pressure, it was mildly tender. It didn't feel warmer than surrounding skin. I didn't have any other symptoms. I didn't really know what to make of it, but we concluded it was probably some sort of trauma I couldn't recall.

But then two days later, it worsened into the ecchymosis seen in the second image. The area of central clearing was still raised and slightly hard, perhaps a nodule; it was still minimally tender and non-pruritic. Any ideas?

We concluded it might have been some sort of insect bite. It eventually got better and completely cleared within a week, but I figured it was interesting enough for me to take pictures and perhaps someone will come up with a diagnosis. Also, if you like this sort of thing, I do have a separate website: http://caseoftheday.blogspot.com/ with mystery medical cases.

Both images are by me.