What Causes Delay In Menstruation Other Than Pregnancy?

hi i am 19 and i was on the combined pill for just over 2year but in january i decided to comeoff it. after i came off te pill i diditn chave a period for 2 months, i spoke to my doctor about this and she said that it was just becasue i have been on the pill for so long and it is nothing to worry about. then by the 3rd month (april) i did come on and ever since then i have had regular periods (coming on on the 10th of each month) untill this month (august) i have not come on. i am getting slightly worried as we have been trying for a baby, i have taken a pregnancy test but the test showed negitive? if i am not pregnant then why have i not come on? and is this something to worry about?