Rapid weight gain

When does weight gain happen after unprotected sex if you're pregnant. It's been about a month + one week since I have and ice gained about 16 pounds some how. I don't know if my eating habits are seriously becoming that bad or it's because I'm pregnant. My period is supposed to come this weekend but we will see. I'm also on the pill so idk if that has another reason but last week I was 209 then 206 and now it says 216? Like how is that even possible.... And before the sex I was around 197. Idk what's going on. Help. 

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