@acid well, you gotta remember that she essentially professed her love for him in a bide to stop him from jumping out of a window only for him to seemingly jump anyway (pulling her along with him). She then gets sent to the future, has to deal with god knows what and most likely finds out Yuta's been "jumping" out of windows to have crazy orgies with women he barely knows further in the future.
Yes she lost her memory, however there are still certain things that can trigger a response in you. Nishina might not even know why she's upset, she just knows what they're doing pisses her off extremely. That or she could be faking the memory loss, hoping that Yuta will chose her instead of the future women, but heeeey that's just a theory, a manga theory!