I'm on birth control. how would you know if you were spotting or if it was implantation bleeding? it's confusing.

Don't be confused!: It is NOT implantation bleeding, it is breakthrough. "Implantation bleeding" would require you conceived, which on OCP is not very likely. Implantation bleeding in the luteal phase is 99% an internet myth that (nearly) never happens, though it sometimes describes bleeding later in pregnancy. I hope this simplifies for you, good wishes.