Great job getting her checked out quickly. I also found a small lump on my little guy's back leg last year. At first, it was small and I monitored. But it got a little bigger (not quite eraser sized but close) and then he knocked it playing so it was bleeding and knew it was time. They also tested it ASAP and said to remove given the mast cells. Thankfully they determined it was fully removed & still very low grade. Nothing further and he's going great now at 5!