This does occur to me as well.
It can be very disorienting.

Depending on the nature of the memory, it can be quite disturbing.
I have found that keeping a sketchbook of vivid memories helps me to find an answer as to the true source of the memory in question.

It may be beneficial to begin a story board as a venue to explore the memory.

If you have others in your memory that are accessible to you, and it is not a subject that
would be better left unspoken, then ask.

I know nothing of your dream.
But it seems to me that if one does not possess the information desired, then one must seek out another that does.

Perception is reality.
Even if it was a dream.
The fact remains that you remember it now.
Glean what precious knowledge you can from it no matter the source.