。
She's really angry with me.
Literal
She [topic-は] me [to-に] very belly [object-を] standing-is.
腹を立てる (literally 'to stand up the belly') is the body-part idiom for 'to get angry' — and in the ~ている form 腹を立てている it's 'is angry,' the state resulting from getting mad. Japanese has a rich set of 腹 ('belly') idioms reflecting an old folk anatomy that locates emotion and disposition in the gut: 腹が立つ ('belly stands up' = get angry, intransitive), 腹が黒い ('black-bellied' = scheming), 腹を割る ('split open the belly' = speak frankly, lay it all out), 腹が減る ('belly shrinks' = get hungry).