Hi. Me And My Boyfriend Had Unprotected Sex And He

Hi. Me and my boyfriend had unprotected sex and he ejaculated in me. The back ground information is I am on the depo shot. After intercourse we went to the nearest store and bought a Plan B one step. The pill was taken thirty minutes after intercourse. When I was using the restroom I noticed I was bleeding not even an hour later. Could this be implantation bleeding?