Not that I can think of, coffee can actually help decrease constipation because of the stimulative effect it can have on the bowels.
Do you add anything else to your coffee? Is there any thing else you eat when you have additional cups?
Also, does she just seem to have issues (grunting, etc) or are there actual issues, and when it finally comes out they are hard nuggets? If when she does have a bowel movement, the stools are normal and not hard nuggets, then she isn't constipated.