I'm a sickle cell patient, why is my b12 dropping?

Many possibilities: People with sickle cell disease can have low B12 for the same reasons as those without the disease. People can get vitamin B12 deficiency when they don’t eat enough foods that have vitamin B12 in them. or they eat foods with vitamin B12 and the body can't absorb it. Reasons for poor absorption could be stomach or bowel disease, some medications, or inherited inability to "grab" the B12.

Not clear: It's not clear why your vitamin B12 has been falling. This is not a typical finding in sickle cell disease. Do you follow a vegetarian diet? Are you regularly taking antacids like omeprazole? These are possible causes, and so are some bowel disorders. Do get this evaluated, especially if your B12 is so low as to require supplements.