31M, almost 1 year post op mini neck lift/lipo & persistent swelling, any ideas? (Photos)

I (31M) had a mini neck lift + neck/chin lipo 11 months ago. I've healed mostly well except for one specific spot under the left side of my chin that still has swelling. The swelling feels like a fatty lump w/ defined boundaries. It's mostly stayed the same size except for one week in March when it ballooned up, became hard, and was very painful. I've seen my surgeon several times about it and he has no idea what it could be. Any ideas? It's still sore when I massage it.