Might be ringworm, google it to have a look - itches like mad, it does, and lasts a good 2-3 weeks. The round, pink skin will show for a long time after it's gone, too. You can put OTC anti-fungal cream on it, like Lamasil, or any of the ones made for athlete's foot. It'll help with the itching, but it really has to run its course. The benadryl is a good idea. When I had it (twice now) I covered it with a band-aid when I went out as ringworm's still associated with being 'dirty'. But at home, leave it uncovered so it'll dry out.
If it gets bad enough, a doctor can prescribe Aclovir (sp?), but most times it can be treated at home.