P.M.
I'm so sorry to hear your loss has been so difficult, N.. An unexpected death often shocks the survivors – there was no time to prepare emotionally for the loss.
Shock and grieving can do strange things to sleep and to dreams. It is entirely possible you are still dreaming and don't remember it. But even if you're not, I hope you don't feel guilty for not remembering your mother in this uncontrollable way. Yes, those dreams are common, but not everybody has them.
After my dearly-loved granny died (and her death was not a surprise), I went completely numb for nearly a year, and don't recall a single dream about her for several years. But then I had occasional, and very lovely, dreams in which I got to speak to her. Unconscious processes take It different paths through different people's souls.