Lesions on spine or even brain are not
necessarily MS but
of course it needs to be checked against symptoms
etc. There are better experts than me here!
Saddle anaesthesia is common after sitting for a while, hence the name, but if it persists needs to be very urgently looked into to check the cauda equina nerves at the base of your spine are not being compressed, If the sensation comes back on moving, you're probably okay.