I just bought 4 neon tetra three days ago but I noticed that one of them is constantly chewing and shivering. Eyes turning a bit yellow. Is it just stress or could it be sick?
That definitely sounds like something to be concerned with! Unfortunately I haven't encountered this before, but I recommend using this website to try to diagnose the fish: http://www.fishyportal.com/diag/
It is also worth talking to the fish store - I know some stores offer a 7 day guarantee on their fish.