Your best bet is to tell someone immediately. You need to make sure you are protected in this situation; even if you have/had feelings for this woman, she should not have engaged in sex with you. You are at very different places emotionally and physically, and she has taken advantage of you by having sex with you.
It's also important you tell someone, because you will most likely need to talk about how you feel. The possibility of becoming a father at 12 is overwhelming, and talking could help.
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