What Causes Fatigue And Forgetfulness After Missing Vitamin B12 Shot?

I have taken a b12 shot once a month for 7 or 8 years.I was checked and have gerd.Take acid reducers twice a day and have for years.I am 75 don't take other meds except for over active bladder.I got my b12 in November and forgot dec to go get it am so extremely tired nervous and forgetful can this be caused from missing my shot almost 2 months late? my email is messed up but try petloverlady7@att,net