If i just started the contraceptive pills, had breakthrough bleeding (did'nt miss any pills) and had sex, could i be pregnant?

Not likely, but: Not impossible. Ask a doctor or pharmacist about the timing for plan b. Then, if you miss a period, do a test. If the sex was protected by condom - you're safer, but pregnancy can be a very powerful force.

Unlikely: If your were taking a pill each day, it is not likely that you would conceive. Breakthrough bleeding does not decrease the effectiveness of the pill.