I Am A 52 Year Old Female That In Past

I am a 52 year old female that in past year had a huge drop in energy and stamina, a couple of months ago I had blood work done and the only thing abnormal was my vit B 12 was low, and was put on b 12 injections once a week for one month and then monthly. I also take po vit b12 daily, it does help if I take a lot. but when I don't take the shots I feel terrible and once a month is not enough. why is my energy so low. I am pretty healthy. even my sinus and allergies feel better when I take weekly b12 shots with po b12.