Sometime i feel like my heart is beating in my nick specially when i get nervous or do some activites , why ?

Probably normal: When we are excited, nervous, anxious, or angry and when we exercise, our heart beats harder and faster and pumps a larger volume of blood. The blood leaves the heart and enters some large arteries that travel along both sides of the neck (carotid arteries). These stronger and faster heart beats are often easily felt in the neck, especially in thin individuals or when wearing tight shirt collars.

I see you are : Using thyroid medicine most possibly for hypothyroid state. An excess of medication could give you those symptoms. A TSH level will help with that diagnosis. If normal an ECG and holter test may help to diagnosed any arrhythmia. Any possibility you are having anxiety attacks? See your doctor for a better assessment.