Can i be pregnant or what other reasons can it

Patient: Im so confused at this point! august 19,2015 i was supposed to start my period, it is now September 3, 2015 still no period. I took a home pregnancy test on august 29th it and came back negative. So i waited a few more days to take another test which was this morning September 3, 2015 and it was negative again. I have all the signs of being pregnant! but i don’t know why its saying negative ! I’m not stress or anything i just want to know what else can be the reason for a missed period.

Symptoms: Tiredness , nauseous,bloating, headaches and back pain