3 weeks late period,3 negetive hpts,no thyroid problem,no pregnancy symptom

Hello ,
Hope you are doing well.I have the following problem.I am 3 weeks late for my period.My last period started on 8 or 9 march 2011 and it is more or less normal(say extends everymonth by 2 t o 3 days).This month I ahve not got any till date. I tested for any thyroid problem but it is normal.I have done 3 HPTS and all have come negetive.I ahve not particularly been stressed also.
I really wish to get pregnant.I do not know that can I still be pregnant even after the tests are coming negetive.
And what otehr possible reasons might be there for a late period(I ahve never been irregular).

Thanks to all!