Ugh. What an awful mess to deal with, especially at this time of the year.
Was your oil light ever on while this happened?
NEVER EVER 'flush' an engine. It's a big waste of money. You can use a spray bottle to get a fine mist of water into the motor- if it's hot AND you're wide-open-throttle. That'll knock most of the carbon out of the cylinder walls. If anybody needs to do that, let me know. I put 182,000+ on a 4-cylinder Saturn and never once 'flushed' the engine. You don't need it- don't do it!
As for the initial pinging, the octane level in the fuel is suspect. If you bought the right octane, where did you buy the gas?
Regarding the oil change, who did it? How did they 'test' the pressure? If your oil light didn't come on, it was fine and that's not the cause of the problem here. That said, there's a lot of reasons to change your own oil- sorry if I sound preachy but this is one thing you want to do yourself- the worst mechanic in the shop (or the least experienced) is the one that changes oil. Oil change shops and dealerships, IMO, are the worst places to get oil changed.
You usually can't check the oil in a pressurized motor because some or most of the oil is not in the pan, where the dipstick gets it's sample. You need to wait until all the oil 'falls' out of the motor before you can get an accurate reading.
Lots of other possibilities here- my guess is your oil pan got cracked and the oil leaked out. Poke around underneath the motor and see what looks amiss.
Best of luck- keep us posted on where this goes.