False-Negative? Could I still be pregnant?

Patient: I wish start by telling you all when my last period started and ended. My last period started May 12-May 15 (I am using a Period Tracker App on my phone). My app say I will be fertile on May 19-25, and ovulate on the 24th. I really hope I am because my husband and I have been wanting a baby :)The days we were intimate was May 21st ,24th, 30th, and 31st. My next period should of started around June 7th. I am now 5 days late.On the day of my missed period I took a Safeway brand home pregnancy test, both coming out negative. On the 3rd day of my missed period I decided to invest in a name brand home pregnancy test, and got First Response, both negative. I wasn’t convinced, so I went into a walk in clinic and they gave me a serum blood test. Well the morning of my 3rd day period i felt nauseous suddenly, gagged but not vomit, and dizzy, just a yuck feeling, and a very mild stomach cramp.That evening the doctor calls and says the blood test came out negative. I trust blood tests, but is it possible that it can be wrong?My symptoms4 days late periodInsomniaConstipation or diarrhea has not had a normal bowel movement in a weekCraving cheesebloatingNausea: Lightheaded, dizzy, yuck/queezy feeling, minor stomach cramp, would last for hours in the morning, stop, and and then continued for the rest of the night.ForgetfulnessMigraines, and I hardly ever had migraines. This migraine would not go away by medicine (Tylenol), and throbbing painful.Last night 6/11/2014 a night had cramps thought I was getting my period, but I checked today 6/12/2014, and still no period.

Symptoms: 4 days late period
Insomnia
Constipation or diarrhea has not had a normal bowel movement in a week
Craving cheese
bloating
Nausea: Lightheaded, dizzy, yuck/queezy feeling, minor stomach cramp, would last for hours in the morning, stop, and and then continued for the rest of the night.
Forgetfulness
Migraines, and I hardly ever had migraines. This migraine would not go away by medicine (Tylenol), and throbbing painful.

Doctor: