more zxr250 dramas

Hey all,
just want to know a few things about why my bike is running abit funny.
1st, i took my muffler of to give it a spit shine and before i took it of the bike was running fine. Now i feel that as i have put it back on and its the same way as it was before, it seems a bit less quieter and over 13,000 rpm dosent quite feel like before, its still abit tight.
2nd, theres more of a whine now when im reving the bike and releasing, kind of sounds like its from the gearbox. might be normal, might not be? i don't know.
3rd, someone took my bike for a ride and my back brakes had very very very minimal pads on them so i only relied on the front, this bloke must of slammed the back brakes and not only did he finsih what was left of my pads but had a go at the rotor itself so i know need a new rotor
Bike just feels like crap, not fun to ride anymore, just dosne't feel the same as before.
HELP!
just want to know a few things about why my bike is running abit funny.
1st, i took my muffler of to give it a spit shine and before i took it of the bike was running fine. Now i feel that as i have put it back on and its the same way as it was before, it seems a bit less quieter and over 13,000 rpm dosent quite feel like before, its still abit tight.
2nd, theres more of a whine now when im reving the bike and releasing, kind of sounds like its from the gearbox. might be normal, might not be? i don't know.
3rd, someone took my bike for a ride and my back brakes had very very very minimal pads on them so i only relied on the front, this bloke must of slammed the back brakes and not only did he finsih what was left of my pads but had a go at the rotor itself so i know need a new rotor
Bike just feels like crap, not fun to ride anymore, just dosne't feel the same as before.
HELP!