What Causes Not Getting Periods After 42 Days?

hi, i m 24 yrs, its 42nd day i haven t got my period, i had an abortion an year before as i had underwent open heart surgery and was weak to carry pregnancy... i had taken 3 urine test using the pregnancy test kit but all the 3 times it showed negative..... i have irregular periods which is now regular after my operation... Please help me... As i m scared... Thank you.. awaiting for an answer...