I don't want to alarm you, but this happened to me once. I went on holiday and came back and my horse had completely changed. Every time I went near him he became a quivering wreck and quite dangerous too. I was really frightened because my first thought was that he had a brain tumour (although for a 4 year old that would have been unlucky). The yard owner where I kept my horse had gone away for the weekend to a show and had left his (at the time) head groom in charge.
I later found out from another groom that the head groom (a really twisted girl) had been beating my horse and had even stuck things into his anus. WHY? Because he wouldn't stand still while she pulled his mane.
What a freak. Needless to say when I found out all hell broke loose. Now the horse (nearly 5 years on) is a changed horse forever, he gets really stressed when you try to get near his tail and goes mental when you touch his mane.
I hope you can get to the bottom of this, and quickly... x