Possibly pregnant, husband has had a vasectomy..no period?

My last period was at the beginning of Jan. 07 I probably ovulated around the middle of the month. I haven't been paying close attention to this, since my hubby got a vasectomy about 10 months ago after a birth of our baby. I haven't gotten my period yet, but I had a spot of blood on the 31 and then the next couple of days nothing. I have took a hpt and it was negative. It is freaking me out, I have never been this late. I am going to test in a few more days- what do you guys think? could it have been implantation blood?? ANYBODY ever heard of pregnancy after a vasectomy??