2001 Ford Mustang Engine Problems

Today, as I shifted from Drive to reverse, the car shook as if I had hid massive bumps, but it was a moderately flat surface. My car didn't sound very good after and I could feel the difference in the way the car took off. When I got home, my car sounded like it was dying (very low RPM's, about 700 or so to be exact, unless the gas pedal was pressed), it just sounded like it was ready to kick the can. I checked the oil because recently I have done an expert job at forgetting to check it, and to my surprise it was the lowest it has ever been. I even opened up the fuel cap and shined a light inside and I could see next to no oil in there. Could it be the lack of oil that made the engine shake and rattle, or could it be something more extreme such as the transmission?