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I've looked online for info about this, but haven't really found anything of any use, so I figured I'd ask here. My question concerns becoming pregnant. And just when I should be worried if I am or not.

I had sex for the first time with my boyfriend a month ago, he used a condom and about a week later I got my period, so here it is a month later and my breasts are really sore and I have no idea why. I'm figuring that since I got my period soon after having sex that there isn't a possibility that I could be pregnant, and I feel really kind of silly asking this, but I just wanted some opinions and such...feel free to tell me that I don't know anything about this, because well, I don't. But thanks in advance.