Is it normal that my bum cheeks vibrate when i sit down?

Are you British? Do you have big ass?

R U A SECRET ROBOT WHO IS GONNA DESTORY THE WORLD? IF YA, I'M REAADY WITH MY LASER GUNS! CUZ GUESS WHAT! I'M SECRETLY A FREAKING AWESOME NINJA!

Ramdom... ya I know...

when you excesrsise to much or alot you sometimes get random spams their annoying but ya

Um did you fart ?
but i kindve get random spasms in my arms for no
reason , I hear you get those from excersice

Bulging/herniated disc in your lumbar spine, pressing on nerves would be my guess. I have spinal injuries ranging from cervical, thoracic and lumbar and have had these symptoms since the car wreck.
See a doc and they'll probably get you to have a CT of your spine.
Good luck.

It's just your nerves firing off randomly

that's a cool party trick

Pulse?

Have you ever had that thing where your mobile phone is in your shirt pocket and you feel it vibrate but when you go for the phone, it isn't vibrating? Something is going on!

I just don't know what it is.