What Causes Delayed Period After Having Unprotected Sex While On Depo Shot?

Hello I got some questions... I was on the depo shot for 9 months and now been off since august that was the month I was suppose to get my shot. It is now dec and still haven't had my period since the first shot... I've been having unprotected sex with my fiance I took a preg test about a week in a half ago and it was neg. Can I be preg or is it the depo messing with my body?