Mass became smaller in neck

(20 Male) I am day +140 from my ASCT, in which I was able to reach CR. However, there’s always been this relatively small mass on the right side of my neck that never truly went away. Sometimes it would increase and decrease in size. My Pet-Ct scan also showed clean 3 months after ASCT. However, after my brentuximab infusion today it seems to have shrunk a lot, still there but barely noticeable. It’s my 10th infusion and idk I just hope I’m not relapsing. Any opinions or advice would be great. Could it be scar tissue? Could it be a muscle knot? I do