very light, very short "period"

Some background...
So I've been on birth control for basically ever and my periods are extremely regular and generally am average flow for at least 5 days. 
 This last month my pharmacy switched my generic from one pill to another (usually not a big deal) and I just finished my first pack of that. I did have unprotected sex during the month but I was taking the pill daily, albeit sometimes a few hours late. 
My period was due on Tuesday and I woke up with what I thought was the beginning of my period on tuesday morning. I noticed that it was very light, but there was some blood. It was again very light on Wednesday. It is now Thursday and I have had essentially no blood. I've also been extremely constipated (unclear if this is related).
Should I be concerned that this is implantion bleeding?? I'm worried... I was going to wait until next week to take a pregnancy test to ensure it would be a true negative... Anyone have any thoughts in the meantime??