Interesting...never heard of it before...so, that means I'll do some surfing.
Definitely you should go to a hematologist to find out what's going on.
Why are you taking sulfasalazine instead of asacol or colazal..etc?
Did you have B-12 checked? Large red blood cells can also mean pernicious anemia (from what I just read).
Did the doc mention if the large red cells were pale?