What Causes Delay In Period?

35days late since my last period hi doc i am 20 years of age. I am 35days late. Since my last period..december 14 but there is something i could not understand. Because the time when i am on my 4days of menstruation december 18 that night my husband and me have intercourse but we did not use any protection..and then the nxt day my urine was so clear there is no blood stain. But the length of my period it takes 7days and the cycle takes 35days..and we continue intercourse for the following days 19, 20 without protection..isn't can be possible i am pregnant? i do hcg test when im on 8days late its negative and i do another test when i am 30days still negative.and this past few days february 22,and 23 we do an intercours again without protection..please answer my problem thank you doctor.