I Have Been Getting Nausious When Hungry Instead Of Hungry.

I have been getting nausious when hungry instead of hungry. I am hungry all of the time. I did not have a period my last cycle, but it is not unusual with my birth control. I am constantly light headed, am having lower abdominal pain (very dull) but fairly frequent. Lots of headaches (again not all that unusual). My breasts are very tender. I Have taken pregnancy tests and they are negative. What else could be causing all of these symptoms?