Alot of people say that crystals change color after use for a number of reasons...the first would be exposure to sunlight, the second would be exposure to certain oils and herbs, and the 3rd would be due to the exchange and sucking up of energies...It was the first stone I had ever bought. When I bought it from my friend it was in a bag full of other bloodstones. I saw it and had to dig through the bag to get t it. When I picked it up I just knew I had to have it. It has been tucked away ever since. I feel kind of sad now that it is losing its color but I still feel the same energies as when I got it.
It could be that it has done its work and needs a rest. I would think about changing it for a fresh one...
Does it feel different when you hold it now? You can usually feel the energy changes if you are in tune with a crystal - I would think about meditating with it and seeing what it has to say.