Early bleeding?

I'm 28 years old and have a 2 year old and I have been on the depo for a long time and about 6 months a go, I didn't renew my shot. I have been having normal periods until now. I wasn't due for a period for another 16 days and just started bleeding lightly. My fiancé and I have had intercousr and have been using the "pull out method", not smart I know, but I don't have any cramps or any other symptoms with it. Could this be implantation bleeding? I never had a period when I got pregnant before and so I'm not used to this. Help??