AF or implantation?

Cara • Expecting our first baby Jan 1st 2016! Married my best friend after 8 years together 10/10/14.
I guess I don't understand the difference well enough. I was due for AF today. I used the restroom this morning and after there was a little bit or darker brown. I have some pressure in my lower abdomen like a weird cramping. I've had brown after AF and at the end before but never on the first day. Anyone know why this might be happening? Thanks!