Why Am I Getting Symptoms Of Pregnancy But Negative Results?

Hi I am 31 and have always been egular with my periods, I have a 13 month old and still breast feed in the morning and at night, my period started again after my son was 9 months and i have returned to a normal cycle. ths month I am late for my period now by around 4 days, I feel like I am getting my period, bad mood, cramps etc but no sign, yesterday I did a pregnancy test and it came back ngative. I have had unprotected sex. I dn´t know what I should do?