It sounds like an MS relapse to me too.
I’m obviously not a doctor or a nurse, this is an opinion only. But to me it sounds like your MS nurse doesn’t know what she’s talking about. As if having a physio exam could cause what sounds like straightforward neurological symptoms!!
You could try getting an opinion from your neurologist? See if you can get an email sent to his/her secretary and forwarded to the neurologist. Maybe an urgent appointment to see him/her, or if you make your email very detailed (ie with dates and exactly what it feels like) perhaps s/he will make a decision about what they think has caused this event.
Then if you want them, see if you can get your GP to prescribe some steroids (which the neuro might recommend). Unless that is, the numbness / relapse started a while ago. In which case it’s a matter of waiting it out.
And assuming your neurologist agrees that it’s a relapse, they might want to review your DMD. Perhaps you should be on something a bit more effective?
I imagine you are worrying about bowels and bladder because of this numbness. See what the neuro suggests, and as Carole said, you could either be referred,or self refer to continence services.