Could I be pregnant?

I had sex with this guy on the 9th Feb and he came inside of me because i was supposed to get my period around that date and I told him it would be safe if he wants to come inside of me as I know i shouldn tbe ovulating... However, today is the 22nd Feb and my period is already late, almost 2 weeks, which is not an unsuual thing for me as I experience period delays quite often, every time about a week late... but this time is 2 weeks... I have tender breasts and I feel a bit nauseous and experiencing headache... I got concerned and I bought a pregnancy test the other day and it was negative... But i dont know if it would give accurate results because it was only 11 days after sex... Could i be pregnat? or is it just pre menstrual syndrome?
Could I be pregnant?
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