Breast cancer margins after mastectomy

Hi everyone, I know it's late but overthinking as always! 
I had a mastectomy just over 3 weeks ago and have healed nicely physically anyway!! I now face another operation next Monday for margins? From what I've read online this is quite rare after a mastectomy or is that wrong?

Does anyone know how long to recover from this op and do you need a drain after? Feeling extremely nervous to have to go backwards again. Also do they retest what they take away from this? Thank you in advance for any reply's xxx