What Causes Vaginal Spotting With Implanon And Unprotected Sex?

hello i have the implant Implanon. And ive been spotting for going on three weeks. And it kinda worries me cause its only at times and me and my husband to be thought i was prego before i got it but took 2 birthcontrol test at the docs and they was negitave. butidk it worries me cause we had completely unprotected sex and it was like the 18. could there be something wrong?