Hi Jesselou,
If your period is overdue then there are a number of possiblities;
1. You may hve O'd later than you think. Is that possible?
2. You are pregnant and pregnancy tests are not picking it up.
3. you are pregnant and may be experiencing an early m/c or what is called a chemical pregnancy (sorry to add it, but it is a possibilty).

So if you are sure of your O date I would suggest getting a blood test from your Dr. Some women don't get ++ HPT, I assume by the time it reaches their urine it is motabolised and tests are unable to detect the quanity they need to show a ++.

I hope that helps
Tanya