I started my period in the third week of my birth control pill cycle, around 5 days early, and have been bleeding for 8 days now. why might this be?

Irregular absorption: Most modern pills are very low dose with little margin for error. Forgetting to take a pill, or anything that results in Nausea, vomiting or diarrhea reduces, for a short time, the quantity of the hormone that are absorbed into the system. The reduction of the usual hormone levels can cause your menses to change. Continuing to take your pills in the regular way generally causes return to normal

Breakthroughbleeding: Bleeding while still on the hormone containing pills is called breakthrough bleeding. If it is tolerable, the pill should be continue, because the issue may straighten itself out over ensuing months. It can also be a sign that the strength of the estrogen component is too low, or that the combination of estrogen/progestin does not work with the particular woman.