From you description, it looks like you might be having a lipoma
Although these are harmless in nature, but sometimes these can be painful.
Sudden increase in size or any associated pain can be a cause of concern.
It is better to consider removal if it is painful.
See a dermatologist
or a surgeon for proper examination and further management.Biopsy
of the lesion can be done if required by the surgeon.
hope it helps
Dr Geetika Paul.