So this is for all mothers or expecting mothers: I am 21, I have a fiancé and we had unprotected sex…

So this is for all mothers or expecting mothers: I am 21, I have a fiancé and we had unprotected sex while I believe I was ovulating (we are trying to get pregnant). I say I believe because my period for the last couple months has been slightly irregular. So, my period is now officially late but i had sex on the 23rd, 24th, and the 25th of September. I have been experiencing some nausea, fatigue, frequent urination, headaches, cramps, my breasts are so tender and swollen and a few other minor things. I took a pregnancy test today which is 15 days from the time I had sex but it was negative. Is there anyway I could still be pregnant or is it pretty much a negative