My Girlfreinds perido didn't come last month, we waited a week and a half before we took a pregancy test, it came negative, we took another one a week later and it came negative again, she has no symptons of anything, no pain at all, we are going to the doctor this Friday, any Idea what it could be?

There are several reasons a period could be delayed.

Stress, hormonal imbalance, diet/weight change, etc. Home pregnancy tests are fairly accurate if used correctly, but there is always a chance of failure. The doctor will most likely do an exam and/or draw blood to check for a multitude of explanations.