She isn't resistance to pain manipulation, she feels pain but has a high enough tolerance to not scream... Well she did scream when she was being burned by Hazel.
Pain Manipulation, "
Pain Manipulation is the ability to control the pain experienced either by another, or by the user themselves, most often in the form of inducing or intensifying it. Used in such a way, pain manipulation can debilitate, distract, or incapacitate an opponent that the user would not be able to defeat normally, and in some cases, strong pain manipulators may even kill their target outright, usually through shock.
Some characters accomplish this through
manipulation of the nervous system, while others may accomplish it by means of
mental assault. In such cases, it can potentially delve into psychological harm, and particularly powerful users may accomplish it by targeting the
soul itself."
She does not resistance anything here, her sense of pain is likely the same as everyone else and just doesn't react to it. Unlike Hazel who's Semblance shuts off his sense of pain, making him resistance to pain manipulation.
Do we assume anyone who doesn't react to pain when injured to be resistance to pain manipulation? Because there quite a number of characters who do that.