Is it a seroma? Underarm bulge getting worse during radiation..

Dear all, sorry if this question has been answered elsewhere, I have tried to find discussions using the search function but given up. My surgeon is widely acclaimed at least in Sydney BC circles as being super-skilled and brilliant.  I told him right from the start that I wanted a mastectomy and not a lumpectomy, and that I did not want reconstruction. There was a double tumour and lots of affected lymph nodes so it was just as well I was having the mastectomy from the get go. The scar was very neat and healed up quickly but a large bulge of tissue remained at the end of it, at the underarm. It looks horrible and I hate it dangling there like a piece of flabby melon. When I saw him for a three-month checkup I asked what he could do about it and he blithely commented that it was too soon yet to do a reconstruction as I am yet to finish radio. What??? Now I recall that he said much earlier - when the drains were being taken out I think - that he needed to leave the extra bit of flesh there or it would be too tight for the reconstruction. What could I say? It turns out that he fully expects me to have a reconstruction, and he believes that even though I said I didn't want it I would change my mind. Do all surgeons think they know your mind better than you do? I have gathered that the cost of reconstruction as a private patient is very high, even with private medical insurance. Is it true that this is where the surgeons make most of their money?

So now I am three days away from finishing radio and that soggy lump is swelling up, the nurses and radio technicians noticed it too and commented on it. Is this a seroma?  Will it go away by itself? I asked the nurses and they said there is nothing you can do about it. They said I should tell my radiation oncologist. What can he do? Does this mean I will have this horrible lump forever if I don't have reconstruction? Can I ask the same surgeon to fix it up so I am properly flat? Or do I need to go to a plastic surgeon? And if I do that, do I have to tell my original surgeon? It almost feels like being unfaithful.  They have us mystified, that's for sure. And our lives are in their hands. Any insights?