That happens to me a lot, especially in the evening, or before bedtime. It's a bit like the feeling of deja vu, except that it is an actual memory, like just walking down a street or something, and it's super vivid and real feeling. I've never thought to ask my T what this is because I just assumed it was a part of PTSD and DID; with neural pathways and memory storage being different from others, and because it is very much like a PTSD flashback except it's not a negative memory; it makes me feel really good.