I have crohns disease with narrowing in small bowel.  Started off at 40mg now am down to 25mg (the lowest i have got to so far). I am also on azathioprine.  
I have never had diarhia, just vomitting, stomach pain, bloating, anemia, fatigue and since returning home from being diagnosed i have been sick 2 weeks ago which i put down to a bug as my 5 year old had sickness bug the week before.  
But last night i felt really bloated and was feeling sick on and off through my sleep.  At 6 this morning i woke up being sick.  I do have stomach pain still everyday some days worse than others but nothing compared to what it was before i was put on meds.  My stomach does make alot of growling and rumbling noises.  Getting a little confused is this a sign of my symptoms returning?  Should i phone the gastro clinic and tell them i have been sick?  Anyone else have similar experience?  

I would.  I started on 40mg too and noticed smyptoms as you describe starting to come back around the 30 to 25mg time and am currently on 20mg.  I haven't been sick yet but a few times i've had the 'excess saliva' in my mouth which if swallowed (and sometimes even if not) I vomit, thankfully i didn't those couple of times but the stomach pain, tiredness, anemia, rumbling etc are all coming back, though the tiredness etc i suppose never really went away and won't for a long time i'd imagine.  I'm starting the Aza this week which is scary but needs must i suppose...  It is very disheartening when just as you start to feel half normal (minus the pred side effects of acne and huge appetite-in my case a bonus as i could do with the weight) the crohns rears its ugly head again to remind us it hasn't gone away.

My consultant said he expected the symptoms to start returning at about this stage stepping down the pred hence starting the Aza.  Definately ring them, what is there to lose?

I agree with Liverpool, contact gastro.  It does sound like you may need to put up Pred.  Same happened to me (I don't vomit just get D) and settled when I put it back up but came back when I tried to reduce again. I take methotrexate rather than Aza and was also hopeful I could get off the steriods. Now waiting to start Remicade. Hope you feel better soon.