I think my cat is jealous, but don't know why! Please help!!?

I have a 3 y/o male cat, Azriel. I moved back to mums (who have 4 cats) with Azz. I was only there for 2 months, during which, his faeces was runny, and he would go upto 6 times a day! He wouldnt use his litterbox, so the laundry floor was covered in newspaper, which he'd use. I took him to the vets, and they dont what it could be. They said a diet change and also antibiotics in case he had a bowel infection. Neither worked. Azz and I have moved into my new boyfriends place. BF has never owned pets, but loves having Azz around. He spoils him, plays, pats, feeds, and talks to him all the time. Azriel now has a tendancy to crap in the bath! It was only once a week or so since moving in, now nearly every day! When i'm not home, its like he spites my new BF. He's always giving evil glares, doesn't like to be petted, but at the same time, is very demanding. 2day, he angrily glared at my BF, then peed on the floor, before strutting off! Hes become an angry little cat, but dont why! WTF??!!!