Possible reason might be initial decay which is not visible outside. Decay can spread internally into the tooth without external blackening. The pain or discomfort felt can either be due to this decay which shows as pain, or sensitivity which shows as sudden pain while chewing. This sensitivity might be due to loss of enamel. I advice you to get an x ray done and check with a dentist to check the initial decay. If this is neglected, it might lead to spread internally and might need root canal if neglected.