I have a '98 Camaro V6 automatic, and tonight I noticed a difference in the way it shifts. Going up a slight incline, I felt like I was barely able to pull up it -- like the car should shift into a lower gear, but it didn't shift until I gave it way more gas. I also noticed that the RPM gauge sort of wavered while it was straining up the hill. I just recently put new spark plugs on, and the only change since the plugs was a Flowmaster muffler which I had installed yesterday. Any suggestions?

Thanks