The most likely explanation for your trembling and palpitations seems to be due to THYRONORM
Other reason could be the hypoglycemia
due to addition of another diabetic medicine in the form of ISTAMET
You can distinguish between these two by continuing your medications.
If you again develop the symptoms,check your sugar levels at that moment.If sugar levels are low,it confirms hypoglycemia and if normal then the symptoms are due to THYRONORM.
In any of the case,talk to your doctor and get the respective medicine reduced.