Help please!

Ali
So my last preiod started in December 25th . I'm not exactly sure if it was the 25th but it was in the 20s. I had sex on Jauary 14th . I took 2 pregnancy tests today and they both came out POSITIVE. 
(To get pregnant , aren't you suppose to have one more period after having sex? Which would be the last period for 9 months)
I'm confused because after having sex in Jauary , I never got my period. Which means I would be 7 weeks pregnant now . But I'm only 3 weeks. 
Help me figure this out please. 

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