Bleeding 8 days before expected period. been on bc for about 10 months. implantation bleeding?

Breakthrough: More likely breakthrough bleeding if you're fairly certain of when the next period is due. In my clinical experience implantation bleeding is inconsistent, unpredictable, and mostly doesn't happen at all - even though it's all the rage on the blogs.

Possible/unlikely: If you are using birth control then the likelihood of a pregnancy is lower. But if you have conceived then some bleeding or spotting would certainly be within normal limits and would quite possibly be due to implantation.