Don't feel stupid!
It's quite a common scenario to have pain from a tooth that has had prior root canal treatment. The pain can arise from inflammation in the periodontal ligament that surrounds the tooth, and this gives the impression that the pain is coming from the tooth itself.
As Sqad suggested, a visit to your dentist is in order. Once you have been examined and a diagnosis reached, suitable treatment can then be commenced. There are many possibilties for the cause of your pain, so would not like to speculate at present.