She didn't dare cry in front of him. In fact, she waited until he was long gone on his way back to his homeland before she allowed herself even a single tear.
Now, though, she's locked in her room, silently weeping.
She's humiliated.
Confused.
She'd felt the lie in his soul more vividly than she feels the earth beneath her feet, and the lie had grown thicker each time he denied it. If Toph had less pride she'd have begged him to explain, but no - if there's anyone more stubborn than Zuko, it's her.
She's more angry than sad.