What Causes Delay In Periods?

i havent had mentral cycle in 3 months.. i just recently had a blood pregnancy test 5 weeks ago and it was negative. I have never missed a cycle since i was 9 and there hasnt been any change in my medication or any new diet or excersise. i am 25 years old and me and my husband are trying to get pregnant.. i am very worried something is wrong.. i have had regular pap smears and im running out of options.. i was to have children so bad because i just want 1 please help!