I'm more inclined to think bug/spider bite as well. My sweet angel kitty Simon had an abscess in his neck that had to be drained and stitched. There were no scratch or puncture wounds (from other kitty's claws) so I was sure it was a spider bite although vet adamantly disagreed with me. Nonetheless he healed fine but then .. well you know.
Glad the bump went down on Lucy and hope it doesn't turn into anything more.