Is Pregnancy Possible Even After Having Negative HPT?

Hello, My name is Megan. I need some advice. My last MP was December 23,2013. I'm usually regular give or take a day or two. I was supposed to start my period January 20th. I still haven't started and I have never been this late! The last time I was this late I was pregnant. I took 6 at home pregnancy tests and they are all negative. My breasts are very tender especially the nipple area and the outer sides. My appetite has increased dramatically and I have the pulling/pressure feeling in my lower abdominal area around the same time every night. My urine output has also increased but I have also been drinking a lot of water lately. I am not taking any medications or have not been under any stress that would cause it to be late. My question is: even though I have had negative pregnancy tests, could I still be pregnant? Please let me know what you think. I appreciate any advice :) Thank you in advance!