Today the muscle from the ear, known as sternocleidomastoid, has swollen. i have to pain in the mouth or gums. only the swelling on the side. ?

Neck swelling: What you describe is not likely a swollen muscle, but a neck mass, swollen gland, or other infectious entity. Whether or not there is pain in the mouth or gums is immaterial. See a surgeon for an exam.

May be TMJ related: Particularly if you've been under a lot of stress recently you may have a TMJ disorder which may be caused by teeth grinding resulting in muscle soreness in the sternocleidomastoid and in teeth and on that side of the face. You may just need a splint or night guard. Either way have it checked out as it may be of a more serious nature.