Assume it's due to leakage under the skin, but never have
had this before.  Wondered whether she'd punctured the vein more than
once (other side, etc.)<<

Yes it is due to bleeding under the skin.  Next time put a heat pack on it and
it will help break up the knot.
Most the times the "knots" hurt like the dickens, so be careful.Very possible
that she went throught the vein, you would have noticed a large ballon type
swelling in the area if she went though the vein.

Shell
( I used to stick people for a living, hated to see that nasty knot, I knew I
hurt them VERY bad.)