This is when someone’s stomach nerves are sensitive.
The vagus nerve is the main nerve that sends signals, and helps the stomach to function.
Often low grade viral infections such as Shingles and the Epstein Barr, create a lot of neurotoxins that mildly inflame the vagus nerve.
So when you drink celery juice, it begins to clean the neurotoxins off the tiny nerve endings inside the stomach lining.
This can cause a gentle reaction.
Also many peoples stomachs are filled with bacteria such as E. Coli, Streptococcus and H. Pylori.
These bacteria die off rapidly which can prompt a spasm, that can be temporarily painful.