Help! What do you do when elderly mum thinks you are in trouble?

My dad died in July following a short battle with cancer. When he was taken into hospital in May, my mum (89yrs) collapsed whilst I was with her and was taken into same hospital with a suspected stroke (turned out to be UTI). They went to the same NHome in June and when he passed in July, mum struggled with accepting his death. Mum has improved since, she used to lose things/wandering to a room where she said the man was Dad/getting muddled with rooms that are identical and wanting to take me to this other room. Now she rings me in the evening to tell me quite crossly to come out of the room next door to hers because she can hear me in there and not to get involved with the man in that room! She thinks I am in trouble with the police and that she's really worried about me. I reassure her I am at my home safe and sound. During the day she is frail but quite reasonable and even talks about this evening behavior thinking it is strange. I think it may be sundowning and the Dr. is arranging an appt with the memory clinic. The man next door was not too happy when she walked in on him unannounced! Has anyone experienced this and did it improve with medication?