Unusual transmission behaviour

[TdotAlero]

I searched the forums and Google and didn't come up with any clear solution.
Here's the story. Car is 3.4 auto.
About a week ago I unplugged my battery for about 15 minutes to unwire and remove the sub from the trunk. Right after reconnecting the battery I loaded my Alero with a bunch of furniture and had to drive around 10kms up and downhill. I pretty much drove like an a$$ due to heavy load.
Since then the transmission has been acting odd. I do city driving in heavy traffic every day for about 15kms each way. Now everytime I take off from green light, the transmission is "lagging" in the first gear. Like as if it was struggling to power the car. I keep my rpms below 2500. It only does the lag when I go from a complete stop, and only in the first gear. Also it tends to downshift when going uphill much earlier than it used to before. No jerky shifting between the gears though.
My question is, should I unhook the battery and then drive it more aggressive to let the car "relearn" the driving habits.
Or is it a sign of my tranny starting to give up.
Edit: 260,000km on the transmission