8yo had a high potassium in blood during virus while throwing up is that because of virus or indicates other desease if no other symptoms after virus?

Impossible to know: but if it is now normal, not to worry! Sometimes extreme muscle activity (including, I suppose severe vomiting) could do this.

Hard to say: The potassium value in many simple blood tests is often high as a sampling error, due to the size needle used to draw the blood/breakdown of blood before the test was run or other factors. There are certainly other issues that can raise a potassium level, but I would never rush to consider them without repeating the test with proper sampling technique.