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When you eat something sour your mouth waters. This is saliva being pumped into your mouth by the salivary glands. Two of the main salivary glands, the parotids, are on your cheeks, near the jaw.
The pain is caused by the inability of the glands to discharge the saliva they are producing quickly enough.
This is most likely to be due to over production of saliva or by slight narrowing of the salivary ducts. It can also be caused by parotitis, which is a low level inflammation of the gland which usually resolves with no treatment.