teething or coming down with something?

Tuesday we were told our little one was exposed to viral croup. So of course, being a FTM, I worry. Last night my son only woke once to nurse, and has been fussy all day with short naps. He did not eat a lot today. However, when I took his temp it was only 99, and he has been drooling and chewing on anything he can get to his mouth. He is not coughing. His cheeks are red/rosy, and his body feels warm. I called the pediatrician yesterday and was told if he gets sick to call and make an apt. Anyone been through this? How do I know what is teething, and what is illness?!