implantation bleeding or from the pill?

[moster]

Hello!

  I'm new to this, but was wondering if I could get a little insight. I have been on the pill for almost 10 years now, and have never had breakthrough bleeding or spotting until two days ago. It lasted for only about a day and was just some brown/blood tinged discharge. It was about a week before I am due to start my period. I am wondering if it sounds more like implantation bleeding or from the pill... here's a little background: 

 Last month my script ran out and my doc gave me a hard time to renew it so I ended up starting the pack 3 days late. The pharmacist told me just to throw out the extra inactive (green) pills at the end of the month and start the new pack on Sunday as usual.  

 So for this pack, I went to the pharmacy to pick up the pack on Sunday, but I was away for the holidays and their hours were different. I started the pack on Monday instead of Sunday. The last time I had sex was Jan 1, and the spotting was on the 13th. How likely is it that I could be pregnant? I took a test and so far I'm negative, I'm planning to test again next week. Is spotting from the pill usually more than 1 day? 

Thanks!!