A few years ago someone filed a return under my name. I don't know how they got my information, but I got it straightened out with the IRS and since then they send me a code number I have to include on my return to show it's really from me. I'm guessing something similar may have happened to you.
The IRS does have a number you can call with questions: 1-800-829-1040
https://www.irs.gov/help/telephone-assistance