Im gonna take a stab in the dark here as my physiology knowledge is only like sophomore level... so i may be wrong.
Ferretin is a protein that stores iron in the cells. When the body recieves a signal it needs to produce more red blood cells it will in turn signal to release ferritin. Ferritin will bind with transferin and be bussed to where rbcs are made, typically the spleen. Once there, iron is used in erythropoiesis. Boom, red blood cells!
In your case, taking aas is the whats causing the body to signal for more rbcs and subsequent rise in ferritin. If you continue using whatever you are using your hematocrit level will likely go up in the near future.
Just curious. What are you using? And where about in that cycle are you at the moment?