There's always a chance. Plan B is not guaranteed to work. It's pretty effective, though. It could make you have irregular bleeding, so the early bleeding you noted could be from that.
If you are going to be regularly sexually active, you might want to consider something more reliable. And if you do plan to continue just using condoms, ALWAYS use spermicidal cream or jelly with the condom.
If you only use a condom and it breaks, you can also insert an applicator ful of the spermicidal cream, jelly, or foam immediately after intercourse (so it's good to have on hand all the time anyway). This will help furthur, but you'd still need Plan B.
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