Need advice!!!!!!!?

I had 3 rabbits until yesterday. A albino male lop earred, beautiful 31/2 mnth old. He was injected against myxostotis 2 wks ago and yesterday was running around full of beans. It wawsn't until the evening that i checked one of the rabbits teeth and went to get the other one and found him in his cage reeking of poo and wee. He wasn't like that before and he was cold. He was covered in poo by his backside, so i washed him, placed fresh bedding and put him back in. Sunday morning at 2am he was dead. The other rabbit i've got survived myxmototis again after receiving injections. Is there a link between the two. Should i mention it to the vet or change over to a new one. My 4yr old rabbit is fine and healthy, so how come he never caught it. I'm baffled and fed up!