I think she mouthed Aishiteru

Im pretty sure she said A-ri-ga-tou on that part, just connect each part of the word with each panel.

Well I think she is not hallucinating, i think that really happen and we are seeing flashbacks or memories that she is remembering that night they spend together, some time had passed since then and now its christmas, something must have happen between the two (something bad i asume) in that time and thats why she is all alone now in the office.