The limbic system is responsible for this feeling. The limbic system is the emotion and memory part of your brain, and is hugely important for how you experience and perceive things. The limbic system has a direct impact on the autonomic nervous system. If you perceive that you’re in a calm situation, your limbic system will impact the rest of your brain, and thus the rest of your body, to make your body act as if it is in a calm situation. The hypothalamus is also part of the limbic system and plays a role in your body maintaining chemical balances. It is also a reason why you feel physically ill.
To give a little bit more detail on a few things: The sympathetic portion of the autonomic nervous system is the the part of your body that makes your heart beat faster, makes you breathe faster, makes your pupils dilate, makes you sweat, and makes you stop digesting food (your blood is diverted to your muscles so you can run if needed). It is the fight of flight response in your body and has a cascade effect on the rest of your body. If your limbic system is going crazy with emotional trauma, it’ll make your sympathetic nervous system ramp up as well. If you just ate and your body all of the sudden stops digesting food, you may throw up.
The limbic system (emotion and memory area of the brain) also directly impacts almost every other part of your brain. The limbic system is smack dab in the center of your brain, thus connects to everything. This is why being in a really intense situation can change how you feel physically and how you even perceive (time slowing down) a situation. One of the important parts of the limbic system is the hypothalamus.
The hypothalamus plays a huge role in maintaining your body’s “natural state”. If you need food, your hypothalamus is the part that makes you feel hungry. The hypothalamus is part of the limbic system, so it is under these same controls of emotion. Under a really stressful situation, your hypothalamus will react with the release of cortisol, which will affect your blood sugar and can make you feel sick.
Now, all of this kind of paints the limbic system as the bad guy, but that’s not really true. The limbic system is also what integrates emotion into what we experience when something is positive. It’s why your mom’s cooking tastes better if you have fond memories of her. It’s what makes your heart flutter when you’re in love. It’s what makes you remember things. It is even the reason why a truck horn can go off in the dead of night and you won’t wake up, but when someone whispers your name you will.