I don’t think MS is predestined in any of us from birth, because studies of identical twins show that where one has it, the other does in less than one third of cases. Therefore it cannot be pre-programmed from birth, or genetically identical people would always both have it (or neither).
However, I personally believe the disease process has already started long before any symptoms become apparent. There is no evidence that having children (in women, of course - no idea about the men ) is a risk factor for MS. There is evidence that pregnancy tends to be protective in women who already have MS, but it’s common to relapse not long after the birth.
Overall, having children neither hastens nor delays the progression of MS, nor changes the number of relapses. What you gain on the swings, you lose on the roundabouts, so to speak.
So I think it’s highly probable that you did have MS before even becoming pregnant - but not in a way that showed itself yet. If anything, being pregnant may have delayed the onset of symptoms for that nine months, but then you had a sort of rebound relapse.
I do NOT believe having a child caused your MS. But I also don’t believe Fate dealt you an unalterable hand at birth, that meant you were always going to get it.
We don’t know what causes MS, but we do know it isn’t having babies.
I think it is a combination of genetic predisposition (a few dozen genes associated with risk have already been identified), and one or more environmental factors such as a childhood illness (but which childhood illness has never been identified, despite multiple theories).