I had sex with my girlfriend on 7th ,15th and 20 th of November,2014 and her natural cycle of mensuration is from 23rd of every month ,but this got delayed this month .
I am really facing lot of headaches over this and on 7th it was her first intercourse.
We have checked on 2 occasions by using pregnancy test on 17th and 23 rd of this month and both times result is negative. Can anyone explain why this happens?
And what should I do
One more thing she is quite irregular in her periods

You can not test till she is 14 days late, so testing too early.

If periods not regular, she may have to wait longer but it is good time to start to test

As well, if you're not trying to conceive, it might be a good time to start looking at birthcontrol.