Hi welcome to Health care magic forum.
Thanks for choosing H.C.M.F.
As you describe it appears to be a lipoma
, with rapid grouth at once so it might have elicited pain in the first 2 days , and subsided. It may be a boil but the pain should lot subside inthat case. No history of injury
so it is not a bruising.
I advise you to consult a surgeon for diagnosis and treatment. Because it is a grouth of quick progress you should be cautious of it.
Manual examination may be suffecient, but if necessory the doctor may order for M.R.I. besides other routine tests for confermation.
Wishing for a quick remission.