I think she's overreacting too, BUT I also understand where she's coming from.

How long have you guys been together? If not that long, it's easier to understand that reaction. If you've been together a while, then she not only has insecurity issues, she has trust issues (the two inevitably intertwined).

You already apologized. I agree that flowers are lame (they're only nice when they're a complete surprise). There's nothing you can do, except insist that you don't want the relationship to end. If that's not enough for her, it's not enough.