Thanks for posting,
From your description the lump may be due to a benign tumor
such as a cyst, lipoma
etc or could be a malignant mass. To answer your question, the best specialist to see right now is a surgeon (visceral) who would perform either a biopsy
or a complete excision (depending on characteristic of the mass) and send to the pathology laboratory for analysis and determining of the actual cause of the mass. Even on physical examination the surgeon can determine which mass it is.
can also be a specialist to visit, either will help (oncologist might refer you to the surgeon for excision biopsy before managing the condition later).
hope this helps