Sudden bald spot?

A.

I had what I thought was a pimple on my scalp for a week or so. Today I noticed a completely bald spot. I have a dr appt next week but I'm going to call tomorrow and try to get in sooner. I'm a teacher and I'm hoping it's not something contagious. Has anyone experienced anything like this? Ideas of what it could be? To be clear I'm seeking medical attention tomorrow first thing. I'm kind of freaked out.