Sorry to hear that @CeeCee ..... It is probably more fluid buildup - tho you say it is 'hard'? Did the previous Seroma feel 'hard' before the surgeon drained it? It does come back sometimes.
Hmmm - shouldn't the Breast Care Nurse be available from tomorrow? Can you just message her with what you've told us & see if she gets back to you.
I reckon you should ring the surgeon's office first thing tomorrow morning & see if you can see him earlier - cos you'll just be worrying about it anyway :( .....
take care & all the best - I hope it is dealt with sooner than later xx