welcome to HealthcareMagic
The pain in breast is due to milk flow in your breast.Thyroid
medicines do have few adverse effects like sweating ;headache;flushing;palpitations;nervous tremors.As for the cold
& shivering please see if you have temperature after the shivering is over.Presently you need to get a thyroid & blood sugar levels test done & consult your doctor who started on Synthroid
;after the reports he will be able to advise better.