Okay it is highly highly unlikely that it is a miscarriage(i work in the healthcare field) because the pill stops ovulation. Okay so whenever you miss a pill and take two the next day, your body sometimes gets a little whacko, and it can be related to where you are in your pack of pills. In a triphasic pill there are many different strengths, so while you may miss one pill one week and not have any problems, the next month you may miss another in a different week and have bleeding problems.
Second even if you did ovulate when you missed that pill, it would take another 2 weeks to actually become pregnant and implant, so a miscarriage is actually ruled out. From the time youovulate it takes 7-12 days for that egg to implant into your uterus, and you are on the pill!!!! Sonit is highly unlikely you ovulated, and highly highly unlikely you are having a miscarriage. Abnormal bleeding is normal after missing a pill.