Who do I contact to see if my identity was stolen?

I've looked at my credit scores & seen who I owe money to but I've heard that it's different, depending on where or who you owe money to & I only saw one score (apparently we have 3?).. My question is that it asked me if I had a mortgage in my name in Sept of 2006 & how much my mortgage is now.. In sept of 06 I was just 18 & had never bought a house or anything.. so who do I contact to see what that is about & if my identity was stolen?