Yes, almost exactly as was your case. I reported for my first rad treatment (a few days after the mapping) and had a lump in the axial area of the side to be irradiated. My radiation was cancelled and I went to see my surgeon. She need aspirated the lump for a biopsy and it turned out to be a seroma which is a pocket of serous fluid that is pretty common after surgery. She drained it and it filled back up a few days later. I was referred to a physical therapist for lymphatic massage and with regular massage on the seroma to disburse the fluid, the lump got smaller and smaller. If I really poke around for it, I can still find it, but it isn't a problem and doesn't cause me any discomfort.
I also recently had a 3 month follow up exam with my medical oncologist. She told me that I had a slightly enlarged lymph node in my groin area. She said lymph nodes can temporarily enlarge for all kinds of reasons and that she would just check it next time (in 3 months) to see if it was still enlarged.