I feel my left tonsil is swelling how can I be sure?
When I sense my left tonsil I feel there's a tiny ball like thing what is it?
Can it be swallen just from one side ?

Your tonsil can be swollen on one side. I would take a close look and see if there is any white dots on the tonsil. If there is then you have tonsillitis and would not go around anyone because it is extremely contagious.