Lolly, you poor thing - I can sympathise with you! I also had no pain from the scar, but the swelling was really awful - and just like a hot water bottle, it would move from side to side as I did certain things. I have had mine drained several times, although I know some hospitals don't drain them - but some ladies at the seroma clinic held once a week have had vast amounts of fluid and were in great pain, some of them having had their mastectomies a long time before mine.
My oncology physio told me last week that I ought to have it drained again, so I'm going to turn up tomorrow to see if the bc nurse will drain it - they told me last time that it would probably be my last draining, and it has filled up a lot more slowly than previously but is now a nuisance.
I urge you also to keep up your exercises to keep your shoulder movements loose - it can hurt when doing them with a seroma, and I couldn't do all of mine until after draining had been done, though I did the ones I could do more often than recommended.
Stick with it, and ask one of the bc nurses' advice; your hospital might not normally drain seromas, but if it's causing so much pain and lack of movement, they probably do make exceptions.
Good luck and hope it's a little better soon.
Hugs, Jo x