She may be having demenita from losing you grandfather. This may or may not involve actual Alzheimers. There are many tests and scans that can be done to better determine her diagnosis.
Your grandma may indeed be just grieving. At her age some memory lapses are indeed to be expected. In the mid to later stages of Alz most patients do not remember family members, names, or even where they are or have been. Even when told these things, they usually ask again minutes later. Bless you for caring. Maybe you could make a visit, and see for yourself her condition. She may need some time to adjust to her loss.When my dad lost my mom after 50 years together, he never got over her loss. Sometimes we wish they could go together. Perhaps she needs visited by a home health nurse, or an assisted living facility for a time to help her get thru this.