I stopped getting my period the first time I got sick and didn't get it again for about
10 months. I still don't know why it happened, but I had lost about
40 lbs and was 93 lbs at my absolute lowest in the hospital so kind of chalked it up to that. It could have been from that, or all the medicine, or the Crohn's. I asked all my doctors (my GI, my primary, my gynocologist, etc) and they all had the same answer of "if you're definitely not pregnant then just wait, if it doesn't come back we'll figure it out then, etc". In my experience, my doctors don't communicate on any level so it's tough to get answers for stuff like this. My GI dr wants my insides to stop being inflamed, my primary doesn't want me on prednisone since it is eroding my bones, etc, etc.
My periods tend to stop when I have a flare (I have had about 4 of them now in the past 4 years) and they just come back when they do. I used to be on b.c. but stopped since at that point nothing was being absored by my body and they would have stopped working anyway. I did read something recently about how b.c. pills affect some people with Crohn's, kind of like smoking or dairy, etc but there isn't much research behind it.
Sorry I don't have an answer but I did go through the same thing! I know it sucks. :(