Implantation bleeding on the pill?

I'm a female in my early twenties that has been taking Kariva/Mircette for the past 7 months (due to unbearable cramps). Until recently I had been taking it at the exact same time each morning. More recently I have forgotten it until a few hours later or up to 12 hours later sporadically (maybe 5 times total in the past few months) due to life becoming hectic. I am in a committed long-term relationship and we are both STD free. While we do not use condoms, since I am on the pill, we make sure that he does not ejaculate inside of me. I am aware of the risks of pregnancy using this method.

On December 12th, I woke up and found significant amount of brown discharge upon wiping and in my underwear. We had sex the night before, and a few times in the previous week but not between the 30th of November and the 3rd of December. As the day progressed there was no more discharge. Again the next morning I woke up to find a brown discharge although a bit less and there was no more during the day and after that it stopped. Since starting on the pill I have not had any types of "withdrawal bleeding".

It has been almost one month since this happened and I am not experiencing any morning sickness, breast tenderness, or unusual fatigue. (Oh, and I made the unfortunate decision of skipping my period this month by moving on to the next pack of pills - was supposed to start placebo pills on the 27th of December. So, I don't know if I would have missed my period. Can I start those lower dose pills at any time to see if it induces bleeding?)

Could this have been implantation bleeding? Or, is it more likely that it is from a hormonal change due to being on the pill and having had sex the night before or some other cause?