OK, so the vet probably gave her an antihistimine injection as one of the 3 injections. It sounds like an allergy of some sort as it responds to the piriton. I am thinking that her immune system may need to settle down, maybe you need to put her on piriton for 2-3 days to try and get the histimines levels back down again...that's just my thinking anyway but I am not a vet and really I advise you to ask your vet for her/his opinion on this idea. I think you also need to look around the house and garden and see if there anything there that could be triggering this off - anything new in the house - change of cleaning products, washing powder, anything. Also, did you use any flea/tick treatments on her just recently?