Not sure how warm it is in your window sill, but can you describe the appearance of the green skin on the older pads? It is hard to see but some look a little wrinkly. If they are it may indicate that the cactus is a little more thirsty than your current watering regime is accounting for.
I was just scolded by a long time member of the local cactus and succulent society, who came to check on some weird malady affecting a couple of my big Agave weberi, for not watering my prickly pears enough to the point of them wilting... so while I suspect Baja is right and that you are seeing a lean towards the sun, I can tell you, from experience, that when a PP wilts it also starts to lean and pads will started to bend over.
Since here in AZ we have had incredibly dry year so far it was not really a surprise and a few days of deep watering mostly resolved my issue. If your PP looks wrinkly you would not necessarily water more often, but maybe a little more thoroughly, by sticking it in a deeper container and watering it deeply until a good amount of water had run through the pot into the container before moving it back to its saucer.