Why is there a pimple on my leg with a red ring around it?

This morning I woke up with a bug-bite looking bump, with a thin, red rind around it. I didn't pay much attention to it and I just ignored it. Then like 20 minutes later, the bug-bite bump turned into a bulls-eye dot and the red ring was still there. Then another 30 minutes went by and I looked at my leg. I saw this big, yellow pimple in the center of the ring. Does anyone know what this is?