Could be serious, such as a broken ring or valve issue. Also, could be relatively minor. Might have gas seeping into the crankcase. How full was the oil when checked? Did it look like it was thin and runny, like diluted with gas? If so, change it and put the correct amount of oil in the engine. Run some fuel line cleaner, such as gumout or SeaFoam in the gas tank (about 1/4 cup). That may help the issue of the gas flowing thru the carb and into the crankcase.