First off I am so sorry for your loss.
After I had my daughter I was told by my primary doctor that after a pregnancy you have to give your body 6-8 weeks to fully recover and "go back to normal" before trying to lose weight by dieting. i started trying to lose too soon after I had my daughter and it did nothing. I am not sure if the 6-8 weeks would be true in your situation, but there is a good chance it is. If I were you I would give it a little more time. Maybe watch what you eat, but don't start the true diet until your next period. maybe you body just needs more time to adjust.