Why all of a sudden I get bad oil/gas fumes inside of the car?

It definitely needs to be checked out as it has it's potential hazards. If it's from the engine compartment it may be oil dripping on to the hot exhaust system. Do a visible inspection in the engine compartment and place a sheet of cardboard under your engine at night and see if there are any drips, if so are the engine oil, power steering fluid, transmission fluid or antifreeze?