What Causes Pregnancy Symptoms While UPT Is Negative?

Hi, I had been on the Depo Prevera birth control shit for about 2 years.. I was due for another shot 1/21/15 but decided not to get one. I had unprotected sex January 16,17,18,23,24, and 25. I m experiencing symptoms of pregnancy already, and it is only the 28th. (Nausea,lower back pain, frequent urination, eating alot) and today I noticed a faint linea nigra that was never there before. I took a pregnancy test today, and was negative. Is it possible to be pregnant this soon, but my hormone levels be too low for a test to show positive? If not, what am I experiencing?