pregnant after the shot?

I had my son in January, then got pregnant in April, had a miscarraige a couple hours before my first appointment in june, and got the shot when I went in for my appointment. We went home and had sex that night not realizing it takes up to a week for the shot to kick in. and now I'm having the same kinda spotting I had with both of my first pregnancies. I know the shot can cause irreagular bleeding but its happening around the time frame it did when i found out i was pregnant both times before. Should I be concerned or is it just the shot?