Well depends on your birth control, it's easier to get pregnant right after your period but the spotting could just be your period starting. Best thing to do is just wait a couple weeks (whether if u have a period or not) and take a test.
Sorry I didn't read correctly above forget about the part about your period starting lol. Hmmm now I'm stumped but still I'd just wait a little bit and take a test.
Yes, because the birth control you took wasn't effective. (As other posters have told you) Your body likely started to spot because of the hormonal changes. You need to wait at least 2-4 weeks from the last time you had sex and take a pregnancy test with first morning urine. If you're going to use birth control, you need to take it at the same time everyday.