Is he just overtired?

My 5 month old son has been screaming on and off ALL day! He woke up this morning happy, and then all of a sudden was very unhappy. Hes been eating fine and I can calm him down for a few minutes then he screams again. Right now he is sleeping for the 2nd time all day (Thank God). He hasn't been pulling his ears or anything. Hes temperature orally was 99.8. I have a call into the nurse on call but they said it could be an hour before anyone gets back to me. What do you think could be wrong?