Seroma - unable to drain due to pain

My mastectomy was 8 days ago and it immediately developed what I now know is seroma. Basically it just looks like a B-cup boob, and has done since I first looked at it a few hours after the surgery.

Today the nurse tried to drain it by inserting the needle into the wound, on the armpit end of the incision. She assured me the wound would have no feeling. I assured her it has had full feeling this whole time - it's still red and scabby and sore. She said my nerves are probably regrowing already. But none of my tissue has felt numb since surgery day and the skin around it is more sensitive than when the breast was there. As in as sensitive as the nipple itself used to be. How can it be more sensitive not less?

She put the needle in the wound, and of course it was much too painful, so she stopped. I asked if she could insert the needle somewhere else on the mound (not via the incision itself) because that skin although sensitive isn't raw, but she said no, it had to be into the wound. She then identified a tiny (maybe 2mm) piece of scar tissue that's developed towards the other end of the wound and decided to try to reach the fluid through that. Although the needle was sharply painful, it wasn't as bad as it had been at the other end in the red raw wound. However after a few seconds she gave up, saying that the fluid might be too thick to go up the syringe.

Anyway, I'm back home now still with a "boob" that feels like a water balloon but now with really painful areas on the wound too, where the needles went in. She's given me an exercise sheet to start and has warned me that the fluid build up is likely to get worse as I do the exercises. She's said hopefully my body will absorb some of the fluid, or if the fluid build-up itself becomes too painful, I have to go back to her and she will try to remove it with a syringe again. I don't like the sound of that, given today's experience.

Has anyone else had a not-at-all numb wound a week after surgery? Why am I not numb? Like I've not had enough pain already.

Any suggestions about what I can do about reducing the fluid build-up (or at least not making it worse)?

And does anyone have any tips that helped them with the pain of needle drainage? (I had taken paracetamol and codeine beforehand, but it didn't make this pain even slightly bearable)

Thanks