。
She must be angry at me.
Literal
She [topic-は] me [at-に] belly [object-を] standing is [must be-にちがいない].
腹を立てる ('to get angry,' literally 'to raise one's belly') reflects the traditional Japanese mapping of emotions onto the belly (腹) as the seat of inner feeling. ~に違いない expresses strong certainty: 'there is no doubt that.'