What can be wrong? Tuna is upsetting her stomach. Pretty much, she cannot tolerate it, and pushes it back out. It's true, maybe you are giving her too much. Cats cannot be given too much fish. Keep that in mind.
It'd be better if you bought fish flavored treats from the pet stores, or you could even buy salmon oil, in capsules, pinch one or two, and pour the oil all over her food. Salmon oil is awesome for skin and fur, and cats love fish flavor, and yet this is a healthier way to keep her happy.
Good Luck!
Maybe you're giving her too much? I was told that tuna is too rich to give to cats, but I know they love it so I give mine one little pinch when I open a can and/or the tuna juice. They lap that right up. But I only do this maybe a couple of times a month or whenever I open a can, just enough to give them a little taste.