Could Spotting With Brown Discharge While On Nordette Be Implantation Bleeding?

Hi! I am currently taking Nordette as my birth control pills for three years already. Started another cycle last January 9, on the 11th day, January 20, I experienced some spotting, a light brown to brown discharge. This lasted for two days. Been experiencing lower back aches as well. I had sex last January 14 and he pulled-out. This is the cycle where I wasn't able to drink the pills on time, but I was only late by minutes. Also drank two pills in one day one time. I am worried that this might be implantation bleeding. I am scheduled to have my period on Feb 2.