No periods for about 2 months, pregnancy tests negative?

my gf had her last period starting on 3rd march, aftr which she took an ipill on 12 march,13. After this she has not had her periods till date. We have done 3 pregnancy tests till now, last we did was today only,ie apr.26,13.all tests showed negative results. Thyroid and haemoglobin tests are both normal. And there are no symptoms of pregnancy. What could be the possible reasons for this ? Please guide.