Yep - as @lrb_03 has said - definitely check with your breast care nurse if you have access to one & try & see your GP (or another Dr in the practise if yours is not available) today as many of the specialists/surgeons will be on their Xmas break until mid January - tho some will be 'on call' at hospitals. When my husband or I have something urgent, I just go to the GPs office & say I am prepared to sit & wait until I can see someone ....
Is it hotter than normal? Is it painful? The heat from the radiation should have dissipated by now - if it IS hot, it could be a sign of infection. It could also be a seroma (fluid) that may need draining - but an ultrasound may be requested to check on that.
All the best & I hope you can be seen sooner than later xxx