What Causes Pregnancy Symptoms While UPT Is Negative?

Im tired, i have back pains, stomache pains, moody, emotional, i sometimes dont wanna get out of bed, Me and my husband had sex and that nigh i started bleeding bad and it lasted a while, now that its gone i still have all the same things i crave diffrent foods im full fast or im never full, i am active i walk alot, i took a pregnance test in was negitive so whats wrong with me? Can any one help