Having unprotected sex right after going off the Pill increases your chances of getting pregnant. Some anecdotal evidence indicates that you may even increase your chances of having twins if you get pregnant soon after stopping birth control.
Sometimes, though, there are cases in which the opposite happens—post-pill amenorrhea, in which there's an actual delay in cycling. In fact, it may take several months before your period returns to its normal cycle.
If you are trying to get pregnant, it's always a good idea to see your healthcare provider early in your pregnancy—and even before. He or she can answer any questions you might have and make sure that you're in the best possible health to conceive.