It could be stress, do you have any other symptoms? loss of appetite, weight loss...anything?
I would attribute your stomache ache due to stress, and for this what I would suggest you do is go to a therapist or seek a counselor or maybe a support group for other poeple who are going through the same things as you are. Greif can take a toll on ones body, and it will take time to ease the pain. Im very sorry for your loss, and I hope you find someone to talk to to help you through this, talking about it will make you feel better. Also, in the mean time, make sure you take care of yourself, and get enough rest, relaxation, and eat properly.