If you will indulge me..
I lost my son what will be three years next month. I have grieved every second of every day but every person does this in a different way. My wife, she is just quiet and while I have caught her crying a few times, she does not say much about it. I, on the other hand, have done probably more in relation to grieving and toward his memory than 90% of the people in the world who lose a loved one. My two other son's do not let on a lot, especially my middle son - HOWEVER, I was cleaning his car the other day for him and I accidentally turned the car stereo on and the music that came blaring out was from the son (his brother) that we lost. The son I lost was a singer/songwriter/musician and has albums/CD's out. My middle boy never lets on, but by discovering that was what he plays when he drives alone, I realized that he is grieving and remembering/honoring.
I would bet that in some ways, even your questioning your feelings toward grieving for your mom here, ARE in fact your way of grieving. I could be wrong, but as I said... everyone does this in their own way. Some people get angry, some get sad and cry like a baby. Others, they are stoic and may even appear uncaring. Inside their heart though, or in the deepest hidden parts of their subconscious, they are indeed grieving in their own ways.
Don't let the way you may (or may not) be grieving be a weight upon you. Losing a child or a parent is a weight enough.