What Causes Late Periods With A Negative Pregnancy Test?

Hi I am 19 Years old and I have been experiencing pregnancy symptoms for the last few weeks but I read on the internet that my "pregnancy" symptoms could aslo be a period symptom, I did take a home pregnancy test about a week ago it came back negative but the line was so light you could hardly see it and the in the second window that was called the control window I think there was only a partial line so I think it was invalid. My last period was June 14th, 2010 but I wont consider myself late until July, 29th, 2010 because my period is irregular and some times comes every 45 days. So my question is if its more likely my period causing all this havoc or a baby.