Keep testing negative, but I know I'm pregnant. Help?

I had my last period on January 6th, and I ovulated on January 20th. I had unprotected with my boyfriend everyday of that week. I expected to get my period on February 3rd, but I never got it. I've had negative upon negative test, using two different brands: Clearblue and First Response. Now I'm two weeks late and I have a multitude of symptoms. At first I just thought I had the stomach virus because I was throwing up and I had a terrible headache. But soon the vomiting went away and just turned into nausea. I have strong cravings for dumplings and tuna salad. I had a lower back ache sometimes, and I had cramps early on. My are very tender and sore and my have grown remarkably in the last week. I've been taking two naps a day, and still feel fatigued. The smallest things make me moody, and I've had an increase in discharge. Along with all of that, I just have a general feeling of being pregnant. Please help me. Why am I getting all these negative test results?