Absence of period, HPT negative, ultrasound clear. Why having pregnancy symptoms?

hi im 33 years old i had my period last nov. 10,2012 and thats my last period coz until now i dont have period since then...i did home pregnancy test and its all negative just today i tested again and its still negative..i went to OB she said im not pregnant when she did an internal examination and she suggested ultrasound just to make sure i dont have a poly cystic vaginal syndrome or something but when i did my transvaginal ultrasound everything is normal it said no focal mass, endometrial lining is not thickened....my question is why im having this pregnancy symptoms like im always tired,moody,breast tenderness and my areola and nipple starting to become bigger and darker,always have headache,back pain and abdonimal cramps and most of all my stomach become bigger and its look like a baby bump..please let me know if i am pregnant or is there something wrong with me???