I cant comment from personal experience smeeg, but I have heard anecdotal comments before regarding an attack of hives shortly after they had received a B12 injection.
I would imagine you have arranged an appointment to see your GP/Consultant to discuss with them the side effects you may be experiencing (if you haven't already, you should as soon as possible)
You may find the link below of some value, and I hope you get the situation sorted out soon;
http://vitamins.lovetoknow.com/Vitamin_B12_Inj ections_Side_Effects