It sounds like a Lyme disease tick bite. I would definitely get it checked out. This is what the rash starts out like:
Usually, the first symptom of Lyme disease is a red rash known as
erythema migrans (EM).
The telltale rash starts as a small red spot at the site of the tick bite.
The spot expands over a period of days or weeks, forming a circular or oval-shaped rash.
Sometimes the rash resembles a bull’s eye, appearing as a red ring surrounding a clear area with a red center.
The rash, which can range in size from that of a dime to the width of your back, appears within a few weeks of a tick bite and usually at the site of the bite.
As infection spreads, rashes can appear at different sites on the body. Erythema migrans is often accompanied by flu-like symptoms such as:
fever
Hopefully that is not what it is, but I would definitely get it checked out.