I've never spotted between periods- i've been spotting for the past week. i'm on birth control & use condoms. why would i spot?

Just one cycle?: Occasionally the hormonal changes may cause changes in the quality & thickness of the uterine lining, and in the amount of blood flow & the time it takes to flow out. These things can make one's period flow change in appearance, including having symptoms such as spotting. An ob-gyn doctor can evaluate if it persists or recurs. A urine preg. Test after a missed period is accurate, to rule that out.