Sudden change in behavior

I have two male cats loki is almost three and thor is five months old they always got along but this past week loki has been following thor around with his nose practly up thors butt and he makes a strange mew and then they start ti bicker. Loki is always quite (loki also has something wrong with his back where he cant walk right). What could be causing this sudden change? And how can i fix it?. I love my boys would never want to re home either one

Loki is just sad that Ragnar Lothbrok died in last week's episode of Vikings, and he's jealous that Ragnar is now feasting in Valhalla's great halls with the Valkyries and warriors of old!

Sorry, I got nothin' lol. Maybe Loki notices/smells something wrong with Thor and is trying to draw attention to it?

Haha figured someone would say something about their names. Ad thanks some advice is better then none i think i should take him to the vet to see if there is anything. I live in a small small town and a few cats got feline leukemia and he did get out. Im not sure if its contagious but by a spca worker thats the impression i was given