What kind of bug bite was this?

A couple weeks ago I woke and felt an itch on the lower part of my butt cheek. I started to scratch and felt two fairly large bumps. The felt larger than mosquito bites, so I assumed they were spider bites, but the location prevented visual inspection of them. No pain at all, just a moderate itch, so I changed sheets and tried to avoid scratching them like all itchey bites, but wasn’t that successful.

A couple days later I saw my butt in the mirror getting out of the shower, and was surprised. There were two bright blood red spots about the size of a nickel. They were amoeba shaped, but they had very distinct borders, unlike the gradual red to pink of most bug bites. I was a bit concerned, but since it was 2 days, and there was no problem at all I just waited, and watched and the next day the bump was pretty much gone, and they were noticeably faded, and within 2 more days was gone altogether.

But I haven’t been able to find anything on the net that helps identify the bastard. Anybody know what’s bites gives 1/2 inch irregular, bright red spots with very sharp borders? In Michigan if that helps.

Spider, spider, had a fright,
on your butt cheek in the night?