My Mother Developed A Small Lump Located Between Her Trapezius

My mother developed a small lump located between her Trapezius and her Sternocleidomastoid muscles virtually overnight. After checking multiple sites, my most likely conclusion is that it is either a swollen lymph node, or a muscle strain. However, she received a Tecna shot (recieved in left deltoid) last wendsday that left her left arm in excruciating pain for the rest of the week. She is worried if this is another result of the shot.

Can you tell us what you think?