Hi,
Localised muscle spasm = pt stuck in a pain cycle - you broke the cycle with massage and rotation mobilisations (neurophysiological effects) - patient feels better.
What is there to understand - you did everything right as far as i can tell !
The area L3 is related to the knee therefore is consistent with the presentation.
The reason she was sore initially may be because you broke the adhesions / cause some local damage but essentially changed how things were so she began to feel better.
As we say in australia... "on-ya" which is short for "good on you".