What Causes Tight Scrotum In Children Suggest?

My 8 year old son suddenly has a tight scrotum for 3 days now. Four days ago he was vomitting and had diarrhea. I just noticed it because he was complaining that it was painful and I saw that it was already tight. We rushed to the doctor and to my surprise, it was back to normal and according to him it was not painful anymore. When we got home, it was tight again until now but according to him its not painful anymore. Should I be worried? Hoping you could help me. Thank you!