I wouldn't worry. I was a Security Officer in the Navy for a few years and have processed several guys through the Security Clearance process. If it is a question regarding your past - just make sure you are 100% honest, because they probably know more than what you can remember. It may be though just a routine interview. The process that they go through seems to be different for every candidate and often depends who they have been able to contact, so it might just be a final interview to collaborate what others have said about you.
If it was something that would disqualify you, they wouldn't waste their time coming back to you. Just be honest and open.
Hope that helps.