My question involves name change laws in the State of: ILLINOIS

I recently went to the office of vital records to get a new copy of my birth certifcate. When I arrived I
was told that they could not find my name in the system. I gave my original birth certificate to them and they were able to find me and print a copy. I was then told that my last name was changed in 1997 a month before my 19th birthday by my father, who was never on my original birth certificate. I was then told that I had to have a parent come back to pick it up because I had no proof that I was the person now listed on my birth cerficate. My mother came down the next day to pick it up and the new birth certificate now has my father's name listed on it. My father claims to have no knowledge of ever changing my name or adding his name to my birth certificate. I have received little to no help from city hall, I was given name change forms and sent on my way.
1. Where do I go from here?
2. How can someone change my name without my knowledge if I was an adult at the time?
3. If my father did change my name when I was 18 yoa, How did they let him do it without ever being listed as my parent on my ORIGINAL BIRTH CERTIFICATE?
4. Can I file some type of complaint with the state for letting someone alter my birth certificate without my knowledge?