It is true that if amitryptiline is taken for 8 or more weeks, there is possibility of withdrawal symptoms if stopped suddenly. It is recommended that dose should be gradually reduced over 4 weeks to 6 months. Your dose reduction over 3 months is within this range though you have taken the drug for a very long time.
The symptoms described by you are not normally associated with withdrawal symptoms. I do not wish to tell you the symptoms that are due to withdrawal lest you start feeling them due to suggestion.
I suggest you hold on for a while. Since your nausea is severe, you can take long acting Metocontin Continus 15mg (ModiMundipharma) 1 tablet at bed time for a few days.
Do not worry, there is nothing much wrong with you.