I've not really experienced any blood loss (that I can see) from my Crohn's as my Crohn's is located all in my small intestine.
Before I was diagnosed and was being tested taking my iron really did make me nauseous. Especially in the evening. In fact, I pretty much gave up taking it then.
Now that I am being treated for my crohn's I'm able to take it twice a day with no issues. But I do make sure that I am taking it with food, as advised by the hospital pharmacist as this can help with nausea. I just try and leave 1-2 hours between taking it and eating/drinking something with calcium as this can interfer with absorption.
If you are having more bleeding and nausea when you take it I would speak with your GP or GI then may be able to reccomend a different formulation of iron that is easier on the stomach. I hear that iron in liquid form is generally easier to stomach than tablets are.