Arm pit lump after lumpsectomy

I had a lumpectomy and a sentimental node removal in May, after approx 6 weeks I noticed a lump under my arm in my armpit. 
I spoke with my breast care team and I was booked in for a scan an an appointment with my oncologist, The scan flagged up a seroma from where my node was removed, they tried to drain it but couldn’t, they said it would resolve itself and wasn’t infected.

I would advise you to do the same to put your mind at rest as I was worried sick, 

Hope this helps.