IBS? not sure...

Hi everyone! i really would appreciate some feedback, especially from people who are suffering from IBS or Crohn's...

I have always had severe bouts of diarrhea that send me literally running to the bathroom and also many instances of terrible constipation.  In February, i had severe, severe bloody diarrhea that lasted for about 12 hours.  It was so bad that if i could have left the bathroom, I would have gone to the ER.  I saw my doctor who referred me to a gastroenterologist. They were both convinced prior to any testing that this was Crohn's Disease, as my vitamin b12 level was 141 (very low).  They said that this was an indicator of crohns.  a colonoscopy was scheduled and biopsies were taken.  He said he saw no signs of imflammation but did see spasming of the colon, and made a diagnosis of severe IBS.
He was still concerned about the low b12, so he scheduled an endscopy.  he found a benign tumor on my z-line, but nothing to do with the low b12.  no parasites, celiacs, or ulcers.  so now i have to have vitamin b12 shots every three months for the rest of my life apparently, but no one can figure out the cause.  i feel like something is causing this and should be fixed.  my regular doctor said crohn's could be in my near future.  I also have hypthyroidism and she said that having one autoimmune disorder makes you suseptible to another.
I am extremely concerned as my life revolved around what i put in my mouth and if it will upset my stomach to the point of severe diarrhea.  i am also exhausted all the time, forget about having any type of night life.  im ready for bed by 8pm!
sorry for the novel, i just feel like my doctors arent as concerned as they should be.  something doesnt feel right in my body and something is causing me to not absorb b12.
please please please, any insight you may have will be helpful.
yours truly,
laurie