Hungry but get full fast

I'm 8 months almost 9 months pregnant. I feel so starving all the time but when it comes down to actually eating I get full off of the smallest portion of foods ever. My doctor warned me about this happening during he first trimester but not the last. Is any body else experiencing this during there 3rd trimester ? Or does any one know the reason for this and why and if it's normal at all? I'm starting to feel sick from not being able to eat a meal.