Saturn Outlook overheating

I recently took our Saturn Outllok tot he dealership because of transmission shifting problem. After the service tech diagnosed the problem it was determined that power train control module needed to be replaced. It took 2 weeks to get the part but it was finally completed and I picked up the truck on a Friday evening (dealer is less than 5 minutes from home).
We did not drive it much on Saturday maybe 10 miles total. On Sunday we took it out to run some errands (very hot day). We stopped to pick up a few drinks @ the drive thru and while in the line the warninglights starting going off. The traction control turned off, the engine power was reduced, the a/c went off. We shut it down in the parking lot, and after a brief cool down we limped it home.
While at home I called Onstar for a diagnostic. Code was p1258--severe engine overheating. We have had it towed to the dealership for diagnostic. The vehicle has 52K miles on it. Didn't have an issue before the transmission work was done.
My questions are 1. What can be the cause of this engine overheating? 2. Could this be directly related to the work performed by the dealer? 3. Is it covered under warranty (we have the extended warranty)? 4. Why did it say the oild needs to be changed now when I just had it changed about a month ago?

Thanks for your assistance.