I asked my Aunt who has 12 cats about what she did when she had the same problem as you. She said that it can some times happen. Her 4 older cat stopped purring in the same fashion as your, she took them each to the vet and was told that sometimes older cats just do not purr any longer. It does not mean that they are sick or unhappy it may just be the way they are.
My oldest cat Nikki (I have four cats and four dogs but I am no expert on them
)is around 3 yrs old and is a rambo outdoors kinda cat. She stopped purring about 6 months ago and the vet told us that she was fine and we should not worry.
I guess the best thing to do it keep loving your little one up. If she purrs then hey, she will keep you up at night. If not then at least it will be happy from the love.
If you are still not sure then maybe a quick trip to the vet or even a phone call might help. My vet in St Joesph MO has often seen one of my cats without charging me a full visit unless they had to run tests.