It is possible to get sunken eyes from food poisoning. But it is rare. Usually, people who suffered from food poisoning will experience nausea, abdominal pain and cramps, vomiting, diarrhea, fever, headache and overall weakness etc. And in some rare cases, people may also suffered from dehydration with symptoms of extreme thirst, sunken eyes and dizziness etc. If you suffer from it in case, call a doctor. take care.
Yes, you should listen to your grandma. There are so many such things happening in the world. It has been proved that what caused by food poisoning are not only sunken eyes but also blurry vision. If you are not lucky enough, you may even lose your life coz of that. However, if you get food poisoning, don' t worry, you should calm down and solve this step by step. First of all, when you are sure of this, you need to get enough rest but remember to call the ambulance first. Then you need to drink as much water as you can to prevent from getting dehydrated. You must keep in mind that not to eat any solid foods again while you are waiting for the help. I wish this could help you.