it could be a brachial cleft cyst. might appear small on the outside of the neck but is actually growing on the inside. mine required surgery to remove (still have the scar) and grew to the size of a softball by the time it was surgically removed.
lump right under jaw about quarter in size showed up hr ago and is growing painfull n soft part of neck. just showed up an hour ago has beeb growing since. it is in the soft part between jaw n neck n the middle. when feeling it it feels like a quarter sized ball. it is painfull to open mouth and touching it it is a burning feeling