It is hard to say what the lump is without additional tests. The dimensions of the lump would have been helpful, too. Since the lump is not growing the possibility that the lump itself is a malignant tumor are very small. More probably the lump is an enlarged lymph node. It might be enlarged due to local inflammation in the vicinity. The pain might be related to the pressure the lump puts on the surrounding tissue and perhaps a nerve that passes closely. The doctor might find it difficult to find the source of the inflammation. In the end, they might want to do a biopsy on the lump and the results will tell exactly what the cause for the lump is. But, biopsy would probably come in the end after all the other testing methods are used.