You know that sensation you get in your stomach when the person you are completely in love with looks in your general direction or when there’s only one more muffin left and you’re afraid the person in front of you in line is going to ask for it (other than hunger) or when your being hunted by a lion in the African savanna? Those “butterflies in your stomach” are a response to stress.
When people are stressed, they experience a “fight-or-flight” response. Brain signals are transmitted to the hypothalamus and pituitary gland, which are parts of the brain responsible for controlling bodily functions. The pituitary gland instantly signals the adrenal glands which sit on top of each kidney. Those glands release adrenaline and other chemicals into your blood stream.
The adrenaline causes rapid heart rate, increased blood pressure, and improved circulation of muscle so that you deal with or flee from the source of the stress (whether it be your beloved, the person in front of you in line, or a vicious lion in the African savanna).
Since blood is flowing to your lungs and muscles, less of it is reaching other organs including the stomach. This causes the light, nauseating sensation we all know and hate.