Computer program says I plagiarized and I didn't?

I copied and pasted my Admissions Essay for high school into Grammarly.com 's grammar checker. It said that there were all these mistakes and that I plagiarized my paper! There aren't any sources or quotations (all I did was introduce myself.) What if I accidentally wrote the same thing someone else did? I can't get kicked out of this school... Is the program lying, or did I really do something wrong? If so, how can I find out how to fix it?