Our mind is a powerful, tricky thing
We actually survive throughout our many years by being able to maintain one continuous self-image, despite the fact we are a completely different set of cells than the day we were born.
I think when we rapidly cycle thru weights, the adaption process can lag behind and a slightly fractured self-image can be the result. Some people adapt to rapidly changes images of themselves faster or slower at different times in life based on the level of changes and level of linked emotions.
Many people here, especially over on the Living Maint board and myself included, totally empathize with the mind lagging behind the body! However, no one but you is going to be able to figure out why your mind is stuck feeling fat.
The "cheated" comments strikes me as especially interested. Also, for you what does feeling thin mean? Some people feel thin when they are slim and strong, others when they are more frail, etc.