Sounds like it's vet time, GoldieTheCat. With the lethargy you've noticed, it seems like there's more going on there than just lack of purring. You should at the very least have a blood panel run on your girl and bring in a fecal sample.
Cats are very good at hiding when they don't feel well, so the sooner you can get her to the vet for a once over, the better. If she's sick, you don't want to wait till she's too ill to hide it anymore.
Good luck with your kitty!
Hope it's just a general mellowing as she ages, but when you see multiple changes in behavior like that, it's always a good idea to get her checked out.