early pregnancy symptoms?

How soon do you get pregnancy symptoms??  
I few weeks ago I had sex on Saturday (on placebo pills , with condom, period finished) and I felt nauseous pressure in lower pelvic area and frequent urination 4 days later on the Wednesday. 
I think I'm just stressing... Surely I wouldn't have symptoms that early?? I took a couple of tests and they were negative and I got my period when I started my placebo tablets the next month. 

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