I understand how you feel. I lost my mom in November 2008. I had been following the South Beach Diet and doing quite well. However, while I was grieving all I wanted was spinach pizza and chocolate fudge mint cookies all the time! (very non SB too, I might add) Yes, I only wanted to eat my comfort foods then. Maybe, you will too. Maybe not. I grieved hard from November to May 2009 and then had some other things start me all over again--both my hubby and I lost our jobs.
You have had a couple of whammies. You got sick yourself. Then, grandma passed away so suddenly. I can tell you that the hurt isn't any different whether they have been sick for a long time or briefly. Pain of the heart is the worst kind of pain.
Like others here, I agree that what you need now is to grieve. If you can talk with others, in your family or with friends. Cry. On occasion, the tears still well up for me. It will get easier but it takes time. You can always resume your dieting later. There will always be time for that. For now, allow yourself to feel the loss of grandma. Again, I am so very sorry for your recent and unexpected lose.