Welcome to Healthcare Magic.
It is either a Sebaceous cyst (Epidermoid cyst
) or a Lipoma
. If there is a central blackish dot in the center then is former one other wise it is lipoma. Both of these conditions are very common and can be cured with small excision. It protrudes whenever you make neck movement because of your underlying muscles on which it rests. During movement of head to left side it makes the the neck muscles of right side tense over which the swelling becomes more prominent. It is a normal phenomenon.
Now for your numbness
feeling of the shoulder and arm is related to the compression of nerves deep inside the neck in the vertebral
column. This is not at all related to your pea sized swelling. Sometimes we do get compression symptoms from any swelling in the neck but then it should be around the size of your fist (or a small potato).
So for the small you can visit a dermatologist
/ Surgeon or can leave it as it is.
For your shoulder or arm symptom you can meet either a orthopedic surgeon or neurologist
who can guide you the best.
Hope this information will help you.
With best wishes of health,
Dr Sanjay K Kanodia