I am apparently a "corporation"? Help!

I filed my personal taxes and included all my business information on Schedule C. The IRS is telling me that I am an S-corporation and that I have to file corporate income tax and redo my personal taxes. I don't even know where to begin. One IRS person told me that I am already 3 months behind on filing the corporate tax form so I can anticipate large fines and penalties. I am just a person with one 1099 client, soon to be zero. I don't know what this S-corporation business is all about.