My quaker was self mutilating?

Ok,so my quaker that is 4 years old started self mutilating after about 2 years after I got my dog (i don't know if this is a factor or not) so we took him to the vet and she said it might be a virus O.o so she gave us to give him pain meds and antibiotics after 2 months of him living in his uncomfy state we took it off and he hasn't done it again! :D But I'm still curious to what mite have been the cause... any suggestions?