I read recently somewhere (?) that around 12 months babies begin to realise when they are left by their mummies and obviously they don't know what's happening. I've noticed this awareness in my dgd (9 months).
She watches anyone in the family leaving the house, (school, work etc) but if mum goes to the front door to answer the bell or open the door, she starts to cry.
Your dgd has just lost her mum as she has gone back to work,and it must be upsetting for her. Some children are much more needy than others, my first dgd was terrible, and at 13 she is still a bit clingy!
You can only love and cuddle her, but I know how helpless it makes you feel.