revving, flames

I'm not sure if anyone here would be able to help me but I'm trying to figure out what could have caused what went wrong with my car. I was on my way to my brother's college with a friend and everything in the car was running fine. We stopped and put gas in the car and when we pulled out of the gas station and back onto the highway the car started revving very loudly, it sounded like a racecar is the best way i can put it, and the accelerator was vibrating at a very high speed. Then, there was a very loud bang and then there were flames coming from the back of my car, or possibly up from underneath it. The mechanic said that there was no oil left in the car, but I had just had my oil changed 6 days prior. Is there anybody out there who could tell me what could have possibly happened to cause this?

I would suspect they din't install the drain plug properly. I would check this out throughly as they should have insurance to buy you another engine.