I got a rescued ginger tom (8kg) in 2011. He is between four and six. I have posted about him before, eating cheesecakes and dining out at the neighbours.
Over the summer, he had a very hard time at the vet's. He wouldn't get in the box and then he reacted to the routine vaccination (diagnosed with feline hyperaesthesia) and had to have morphine. All fine within a week.
In the early autumn my neighbours texted me and told me he'd been spending lots of time there. He'd not been eating so much here but had been gaining weight. They wouldn't stop feeding him bloody Dreamies so we decided to keep him in. I am sure even if they would stop, he would find someone else to feed him.
His personality has totally changed since he met these neighbours (and napped with them in their bed 
), or maybe since the vet's, and has got worse since he became a house cat.
He has always been very clingy, always in same room as me, has weird very loud howling episodes when we leave him to go to bed even though he sleeps with us!! Every night it's like he forgets where we've gone, thinks he's been abandoned, then finds us when we call 
He's also got issues with food. He's like a dog - will eat whatever we put down and eats it all in under ten seconds then burps, sometimes voms, and eats that
He's also dominant, will sit higher than us and square up to us etc. When I get in from work he wants to be cuddles with his arms around my neck and he shakes and cries and purrs and kneads me. It's like bloody attachment cat parenting!
Now, though, he's a million times worse. He's on two pouches of whiskas a day as he is overweight (not as overweight as he sounds at 8kg - he is two feet long, massive paws etc). He shouts at us all evening on the coffee table. He attacks things all the time and chases bits of fluff round in manner of something from exorcist. My niece grabbed a packet of his food and he properly squared up to her til she dropped it.
He is obsessed with meals, gets on kitchen counters despite shouts, patrols his food cupboard, licks old plates on the side , comes running from downstairs if a fork chinks a plate, skidding into the room with food in, squares up to guests who go in his places and bites me when I put his food down.
He's driving us nuts. He meows constantly!! What can we do?