What’s Happening?

I have been taking Norethindrone for over a year now. I was switched to a generic form but the same exact dosage of the same exact hormone. I take it pretty religiously, maybe a minute or two late here or there. I’ve had regular periods, and my period actually just ended last week (9/4). Today (9/10) I woke up bleeding/spotting with cramps. I did have sex once while still on my period and three times after it ended. Do you think this could be implantation bleeding? Or do you think it’s breakthrough bleeding from switching to a generic? I plan on taking a test tomorrow.