2000 Pontiac Grand Prix Excessive Oil Usage

Hello, and thanks for reading.
I have a 2000 Pontiac GTP the JUST flipped 115,000 on a recent trip of 320 to destination and 320 back. Just before the vacation I had oil changed, it was about a quart low, said on the reader, but usage was at 77%. Changed oil and all other fluids checked and took the trip. While in cruise control had to excellerate, to hit button to do so and engine light came on, was like that the whole trip. I assessed that to crappy gas...(ALWAYS use premium gas) and when I came back took the car to get readout of codes for check engine light. Said it was all emmisions. Yet all EGR and 02 have been recently replaced, as the Catylitic converter was also. So three (3) weeks passed, new gas, at drive thru "low oil level" light comes back on, then goes off. Checked oil when car was cool, sure enough, 1 1/2 qts low. Gas mileage has been ok, 24 city, 33-34 hwy, but what is causing oil to be used? NO LEAKS visible anywhere? I love this car, and this girl does not want to give it up! Any answers to help? Thank you ~ Ann...Roanoke, VA