In her experience with battle,
Sango immediately learned that not a single soul—no matter how driven—is invincible; weaknesses plague all, e s p e c i a l l y for a human warrior. The risk of injury and death abruptly increases and one can no longer afford a careless mistake.
Physically, she is in her prime, able bodied, light on her feet, and quick to respond in the face of immediate danger, she was not formerly titled the strongest taijiya of the Demon Slayer Village without reason.
So how, how did this slip out of her control so easily? Usually, she is able to restrain her thoughts, remaining focused, solely focused on the task at hand, but during an offensive charge with Hiraikotsu in grasp, an unexpected flashback invaded her mind.
Is her gut trying to tell her something, is he in trouble? His name unfortunately distracts her, unintentionally leaving herself open for a counterattack. In consequence, a vivid burning slashes through her upper body, pain searing through her chest—dammit! Hurry, fall back, Sango! Three second self assessment, heart is still beating, limbs are intact, so fight!
Her weapon is unleashed, propelled by an incorruptible will to survive; when it finally emits its own demonic aura and toxins, eradicating the demon, the taijiya still refuses to relax. As soon as Hiraikotsu returns, Sango senses another presence within range and her left hand rises to cover her profusely bleeding chest.
"There's no use in hiding! If you wish to fight me too, let's go!"