What Causes A Delayed Period With Negative HPT?

Hi, my period is over a week late which is very unusual. My period is never late and is always on time. I took 2 home pregnancy tests last sunday and monday and they were both negative. I then went to the doctor on tuesday last week and his test was negative, and he wasnt much help on why it could be late. I'm really nervous because it been almost 2 weeks now and i still havent got it. Could I be pregnant or could there be something else wrong?