Hey Anonymous, First of all you made a good move to take opinion about your health problems from doctors through Health care magic. Coming to your question I think at your age and location of the lump it should be a simple Lipoma
which is other than cosmetic problems its nothing to worry but I need to know more about it like how big is the lump, is it increasing in size, when you feel it is it rubber in consistency. If you could answer me these I could come to a right conclusion. About your weakness and pain question try to frequently change position, follow a simple daily exercise regimen and you should be OK.
Please lemme know if you have any other concern.