Ok, I'm really confused and the more help I can get the better. Ok, first how long after intercourse dose implantation bleeding occur ?

Now here's my story. My boyfriend and I hadn't slept together in over a month. On Saturday 9-15, we had sex, unprotected, he ejaculated in me as usual. I am on the pill for 3 years straight now, never missing one and always take it around noon, like clockwork. On Tuesday 9-18 I wen to the restroom and noticed reddish brown in my underwear and when I wipe. I have been wearing a pantiliner since then and don't ever fill one, but leaving brownish spots in it. I then had a bowel movement and wiped and it was pink and red like the beginning of my period, the thing is I am not supposed to start for 15 days. I am soooooo confused, could this be pregnancy and implantation bleeding or just breakthrough bleeding ? I am not cramping, but am a hungrier than usual, like when I'm on my period...... Any help would be GREATLY appreciated