It isn't entirely uncommon for circumcised boys to develop adhesions. It sounds like maybe a small piece of leftover foreskin could have reattached to the glans. It's a good idea to check with your pediatrician. Even if it's not an adhesion, a penis is such a sensitive thing you don't want to let it wait.
Some adhesions can cause painful erections after puberty, so keep on top of it. If you don't get a satisfactory answer from your pediatrician, call a pediatric urologist.
Get a second opinion :)