What Causes Bleeding After A Week Of Regular Menstruation?

I am a 19 year old female, 5 feet tall. I began taking the birth control Tri-Sprintec 7 months ago and since starting it my periods have become regular. I am sexually active. About a week and a half ago I had my period as normal, but it was shorter than normal. About 3 days ago I started bleeding again. I had forgotten to take my pill for 3 days, and at the end of the three days the bleeding began. I took the 3 pills I missed right away. The flow is fairly light, as I am only going through about 2 tampons each day and part of it is brown blood, the other part a darker red. I do not feel crampy like I do in a period. I am confused about what is happening. I've been researching online, and haven't been able to find a definitive possibility. Could my bleeding be caused to missing 3 pills? I've missed 3 before and this never happened. Could it be implantation bleeding? Is it even likely to get implantation bleeding just 4 or 5 days after ending a period?