Hormonal imbalances or pregnancy?

In less than a five week span, I had taken Plan B twice as my SO had clearly ejaculated inside. Figuring that it wasn't the best method of contraception, I began taking birth control pills yesterday, which I believed was the start date of my period (it was due between Friday and today). Friday, I experienced brownish spotting, and today (after only a day of light bleeding), my period has finished. This morning, I was terribly nauseated and had to throw up a bit. I took a pregnancy test on Friday that had come out as negative. Could all of this been due to the hormonal imbalances I've introduced to my body this past month, or a pregnancy?