Possible miscariage? Need help

Hi. I'm 17 years old, and have been ON the pill for about a year now.. I had just got off my period (so I hadn't taken any pills for the seven days I was having my period) and had just started up a new pack of pills that morning when I had regular/unprotected sex. A little over a week later (two weeks before my period) I started having spotting. I just graduated high school, so I also wake up at different times in the morning (naturally, now) so I take my pill at different times. Nothing too big, maybe just an hour or two different every morning.. but that's only been since this new pack of pills because I just graduated a few weeks ago. On top of that, a few days after intercourse, I had missed a pill.. just one, but I promptly took it the next day with that day's pill, just like I'm supposed to. Anyway, my spotting started earlier this week, a week after sex, and at first I thought it was just because I missed a pill. However, I've missed a pill before, and that's never happened. My spotting has lasted about five days now and is pretty heavy, almost period like but not quite, and very dark brown- sometimes black. About two of those days I had minor cramps, as well. This leads me to believe that maybe I had gotten pregnant, and this is a miscarriage because I've looked up a lot of the symptoms and they all seem to be describing what I'm experiencing. However, I took a pregnancy test yesterday, and it said I'm not.. but would it detect it that early? and would it detect it after miscarriage, if that's what this is? I'm very confused about the whole situation. I still have another week before my period is supposed to start. I hope somebody can explain this all to me.. :-(