What Causes Missing Periods And Negative HPT?

Hi, I am a 20 year old 5'5 195lbs . I'm 9 days late on my period i took a hpt and says negative when I was pregnant with my now 5 year old daughter the tests kept coming out negative i would take one every week till it finally came out positive at 8 weeks pregnant. Doc, says that the reason for that is because I drink no water at all could it be that I am pregnant.?