Cars not running right....help??

Well I was driving home yesterday from a weekend getaway, and stopped in to refuel....
Before that my car seemed to be running just fine...as soon as I got gas I noticed a horrible pinging from my car, in fact I would have to downshift to 4th on inclines just to get it to not sound so bad.....
Well i assumed, bad gas ???
However this morning I got up and went out to start the car, it started just fine until the choke kicked off. then it sounded horrible, like it was misfiring, in fact it backfired a couple times...you had to pretty well floor it to keep it running. I seen black smoke so I went around and sure enough, a nice black sooty spot below the tailpipe.....
I checked the spark plug wires, thinking maybe one got melted on the manifold or something......they seem fine...
What should I look for??? I'm really not a mechanic chicka so I'm at a bit of a loss....but I don't even want to move it from the driveway til I figure out whats going on with it.
This car is an 02 V6....40,000 miles on it.
Just had an oil change.....oh, I took a look and the oil cap was loose...not sure if that means anything ????
Any help would be awesome!
Thanks,
Stacey

I just checked the oil level...it reads above max .....what does that mean?

I've been asking around....someone told me it sounds like I put diesel in it by mistake? Could this do it? I know I read the pump but....makes me wonder, it was an older gas station.....