The fact that you're not using that Facebook email address appears to have led to Facebook trying to figure out new ways to push you to the system. So, they've taken action by forcibly changing your contact email address listed on your Facebook page from your actual address that you regularly check to [your.name.here]@facebook.com — or worse, [random number]@facebook.com.
What does this mean? Well, if your high school sweetheart finds you on Facebook and wants to contact you, they'll have to do it on Facebook — there's no external option. There's nothing opt-in about this change, and there was no notification about it either: Facebook just decided to swap out emails to try to get people to use their cobwebbed email system.
