Action | Closed to Angel
The thing about friends arriving from different time lines, sometimes information that certain people chose not to share comes up, particularly when that certain someone isn't known for holding her tongue anyway. Cordelia didn't know she was doing anything wrong. She didn't know that she was spilling the beans on something Angel had neglected to be bean-spill-y about, something that could really have used some spillage.
As a result, Buffy is angry and hurt and somehow surprised. Angel knows she cares about Spike. He knows that they had a relationship that ended, but remained a friendship. He has to suspect that Spike and that shiny amulet he brought her is the reason she and the girls survived that battle with the First Evil. He also has to know that Buffy feels responsible for Spike's death in part. Sure, wars have casualties and no one was guaranteed to make it out alive in that battle, but that doesn't mean the deaths of her friends were something she took lightly. Buffy comforted him when he talked a little about the deaths of his friends in his own battle.
Right or wrong, she's feeling like Angel omitted the whole 'hey, Spike's alive' because he's a jealous twelve year old boy. He did show up in Sunnydale, go all Dawson on her and then they had the whole cookie dough speech so maybe he thought that gave him the right to decide things like that for her.
Whatever. It didn't. It's probably pretty clear by the way she storms in his house that she's angry.