After I eat, I feel heavy in my head an very sluggish.

I would experience low sugar spells before the following control habits. I have been on a dietitian planned diet (6-7 small balanced meals) and exercise daily. Immediately after I eat, I feel extremely heavy in my head and very sluggish. It takes 30-45 minutes for the feeling to subside before I feel fine again. Could this be glucose related? What do I need to do?

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.