Just started Bucking and rearing! Help?

I bought my 14.2 STB last year, initially he bucked me off twice, after having a master saddle fitter out he told me the prev saddle didn't fit so I put it down to that. I bought a new professionally fitted saddle and he seemed happier. I was never able to mount from the ground with this saddle as it would slip round. I dosnt slip much while on board though. A month ago I had saddle and back checked by a Physio as he seemed a little twitchy. All was fine. Since then we have done miles of great rides, no issues. When I tighten his girth (slowly he moves his legs square as is bracing himself and rounds his back as if gathering him self up. I stretch his legs out before mounting. Yesterday I got on (friend holding stirrup) and he was fine... Until I asked him to walk, he rounded his back and hopped springing around until I came off, he carried on doing this after I was off. I lunged him and put it down to the wind. �Today I tacked up and lunged him, he was fine to lunge, I got on, same as the day before he stood fine, as soon as I asked him to walk he reared then bucked me off, today I ended up�In A&E. I don't think he is napping as he usually likes to go out and we were in company both times. Today after untacking Hugo he adjusted his legs and dropped to have a wee, he only managed to squeeze a little out, about a cup full! I will be calling the vet in the morning but would appreciate any ideas or similar experiences. The only thing I changed was I dropped his cheekpeices one hole each side. Any ideas will be appreciated.