Why do you have to be off the B-12 for 2 weeks? Is there a cause for your deficiency, other than diet?
Many who are on a vegetarian or vegan diet are deficient in vitamin B-12, since it only comes from animal sources, such as meat, dairy, eggs, etc.
Since your level was so low (89), 3 months on supplements might not have been long enough to get it up high enough to sustain you for any length of time, so, yes, it's possible that symptoms could come back in 4 days...