It’s part of the body’s reaction to the sympathetic nervous system (fight & flight) kicking on… the digestion system (part of the parasympathetic nervous system, rest & digest) shuts off. Sometimes quite violently (puking, pissing, &/or shitting yourself). Other times it’s far more mild; a nervous stomach, and the increased need to use the bathroom.
The digestive system is the 2nd biggest energy suck, after ratiocination (the ability to sit, stand, move… and not just be a human shaped puddle.). Ratiocination gets amped UP when the sympathetic nervous system turns on, and slammed down when the parasympathetic takes over (like when your sick, and even something right next to you seems really far away / too much effort to reach).
Digestion gets shut off when the sympathetic turns on, and also when the immune system decides it needs that energy directed elsewhere.
It’s part of why most people have difficulty eating when they’re under stress, anxious, etc. Their mouths get dry, their stomachs get jumpy, anything they do eat either sits like a rock or makes them feel like throwing up.
The disordered eating that so often comes along with PTSD is largely owed to the increased levels of stress & anxiety.
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Long story short? Makes total sense that you felt sick to your stomach whilst dealing with your abuser, flashbacks, etc.