Implanation bleeding or my period ?

okay so im 19, and my cycle is regular for the most part. me and my bf had on atleast 2 of my fertile days im sure of. i was supposed to get my period on the 16th ... it was a no show. instead on the 18th it was bleeding but not heavy enough to be a period. i put on a pad jus in case (sorry tmi) & the only thing on it looked to be brown, and when i wiped it was like bright red or pink. its not heavy at all and has gotten lighter, today is the 2nd day. i also had menstrual cramps a week before my period was due buh no bleeding. the only other symptoms ive had is lower back aches, nausea (like i feel like ima throw up my food but never do), and headaches ... can someone help ?