I'm paranoid about lumps, Boxers are known as "tumor factories" by many vets. I hope it's nothing serious. It could just be a bruise or fatty lump as was mentioned. My Ginger had a grade 2 mast cell removed from her neck at the age of 2-1/2, she just turned 6 and has been doing fine with no recurrence, so even if it's something like that, it sounds like it's in a good spot to get clean margins, so try not to worry too much. Hopefully the aspiration will turn out to be nothing.