yes, you are right, after you changed the oil, the problem (which was already there) started appearing .. sadly, you have to get the rings, pistons, gasket, etc checked now : (
if you've recently bought the car, sometimes what sellers do is they put very thick oil or STP with oil, it makes the oil so thick that the engine does not produce any sound or smoke, etc., but whenever you go and change it to regular oil, all the problems that the engine has start appearing : (