Why do i get negative pregnacy test?

I was taking pills but not how i was suppossed to, i had my period may 10 i was suppossed to get it aroud the same days this month of june but nothing, i went to the clinic to explained my situation and they took another test, it was negative now they told me to return in 2 weeks for another test,,, this is weird i have never been late on my period so many days but my tests shows negative,, yes ive had unprotected sex so im actually worried of not getting my period y3t.. help please has anyone experienced the same?