I am 34 and on bc. I started to have mid-cycle bleeding on Day 12. Now day 14 and it is still going. It does resemble a light period. What's going on?

Several things. Several things can cause break through bleeding on birth control pills including taking them late or forgetting them, a change in other medications you are taking, a change in pills from one brand to another, one generic to another, or from a name brand to a generic, or a pelvic infection. Without more history and an exam it is impossible to know for sure what is going on. Hope this helps.