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.
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.