Viral infection? kids sick too. this a reoccurrence of something 2-3 mos ago: fatigue temp 35.6 c nausea & cramps mild flank pain flem, sore throat

Could be: It sure could be a viral infection, although the lack of fever is a little unusual. If persists for more than 5-7 days or worsens, you should see your family physician.

New illness: Viruses don't sit dormant and recur. If it's back, it's new. It could be a repeat of the same virus in a new illness, or a completely different one with different complications, like strep. Take the symptoms on their merits and get checked out if necessary.