This is an older post but I found out the answer to this question a long time ago and thought I'd put it here for those who may still be looking for an answer:
It is your blood sugar that makes you so exhausted after you eat. This is a somewhat common hypothyroid problem.
It is known as "Reactive Hypoglycemia."
Before I even knew what it was called, I corrected my problem by eating much smaller meals and never going without sustenance longer than 2-3 hours. I used orange juice laced with the finest unrefined sea salt in between meals to keep my blood sugar from fluctuating. Just a few sips was all I needed to keep my blood sugar levels intact. Then when I ate a larger meal, my body became much less reactive to carbohydrates. I also increased my fat and protein intake. Soon there was no reaction at all and what had previously happened to me for years (crashing with exhaustion after eating) was completely gone.
"This condition, called reactive hypoglycemia, occurs when the body secretes excess insulin in response to a high carbohydrate meal – causing blood sugar levels to drop below normal." (Chris Kresser Jul 23, 2010)