Ellipsis or Emphasis?

Here is the relevant definition of after in the online version of the Cambridge English Dictionary.

following in time, place, or order:

• After (= despite) everything I've done for you, is this the way you treat me?
• After (= because of) what she did to me, I'll never trust her again.

In fact, as you can see from the words = despite and = because of that after does have a causal or concessive use.

Now it might have been more typical to have omitted the negative not:-

I can't believe she'd do that after all I've done for her

But the speaker has instead repeated the negative in I can't. It is a rhetorical device of repetition for emphasis. Here what is being emphasised is the disbelief. It grabs the audience's attention, just as it grabbed yours.