It can be one of two things, either your subconscious wants to hear her again and therefore replays her voice when you're sleeping and you think you hear it with your ears so it startles you and you wake up. Or it really is your mother visiting you and trying to tell you she's still with you.
I've had a few similar experiences. One night I was in my bedroom listening to music and I heard someone on the other side of my closed bedroom door call my name. I thought it was my family member so I opened the door, but no one was there. The only time I've ever "felt" something, was once when I was spending the night at my mom's apartment and one morning I felt someone pressed up on my back as if someone was laying down next to me, I sleepily pushed against it with my back and while I did that I realized no one should be there, so I shot up in bed and no one was there. Later my mom told me she experienced something similar. The apartment was very old and had many elderly people that lived in that building, so we assumed it might have been a previous occupant that once lived in that apartment.