First, have you discussed your symptoms with your dietitian and what has she recommended?
Do you also feel weak and shaky like you are going to faint and break out in a sweat without expending any exercise? Or is the heavy head and sluggish feeling unrelated to low blood sugar?
Sounds like you may be eating too much food at one meal. Without changing how much or when you eat, I would suggest you keep a record of what you eat for the next three days. Show these food records to your dietitian who can evaluate your meals for calories and carbohydrates.