help with rear rear disc nuts hitting the swing arm

Hey guys this is my first post and just wanted to say i love the site. My name is Simon and i just bought a 1991 fzr with 25000k's on the clock.

Any way to the problem at hand, i did a routine check over after buying the bike and found the the chain was a little under tensioned, so i tensioned it up then went for a test ride, as i was riding i felt a knock everynow and then coming from the rear wheel and i could feel the back wheel kick when i felt the knock.

so when i got home and sussed it out i found the nuts the hold the rear brake disc together had been hitting the swing arm ever so slightly

so i am wondering if it was fine before and now it isn't wat did i do wrong, the chain is in good nick and the optimal tension, the back wheel is straight and no crabin sideways or ne thing so i am wondering if it is missing a spacer or something on the rear axel,

ne idea or help would be great, thanks Simon.