My hair fell out after the first time I got really sick - the drs don't know, my GI tells me to go to my dermatologist, they tell me to ask my primary care dr, etc etc - here are some things they said it could have been for me:
*extreme weight loss (about 35 lbs, down to around 98 lbs)
*trauma of getting very sick very fast
*** this was the diagnosis my derma gave me, similar to what pregnant women go through, it starts about 3 months afrer a traumatic event, hair loss mostly around the perimeter of the scalp, lasts for up to 6 months of falling out, peaking in the middle then tapering off.
Once my hair started to grow back I got sick again, went through the same cycle, but it is not as bad this time (then again there is less to fall out, you MUST have a sense of humor about this :)
One thing I unfortunately realized is that doctors treat the problem they specialize in - so while my GI dr might try to understand that it is difficult for a 32 yr old single girl (who has enough problems, lol) to have her hair fall out on top of everything, the main priority for them is getting the Crohn's under control so while they may try to empathize, it has been my experience that I end up leaving the office with as many answes as when I came in.
Good luck and let us know if you get any more info! BTW I forgot to mention that as stressful as it is, your hair WILL grow back, pretty as ever. :)