With the red around it, it kinda looks like a bullseye, which is a mark for a tick bite. But, I would think if it was a tick bite, you would have seen the tick.
It could also be a spider bite. One of me employees has been bitten twice in the last 3 months by spiders. The first time, he was in the hospital because the spider bit him around his private parts! Very painful. This last time, (last week) the spider bit him on the back of his head. The infection is inside his head, but the bite looks horrible.
So, having said all of that, please keep an eye on that bite. If you have iodine, use it. Oil of Oregano is good, too.