Lump under skin in upper arm should i be worried ? just smaller than a marble doesn't hurt when touched feels pretty soft ?

Lump under skin: No reason to worry, but there are many causes for lumps under the skin. It is a good sign if it is soft. I can't be diagnosed without seeing it, feeling it and sometimes biopsying it. See your doctor or dermatologist for an evaluation.

Lipoma?: This sounds like a benign collection of fatty tissue called a lipoma. It could also be a cyst. It is worthwhile having your doctor check to be certain, but sounds ok.