Hi there. Well, I'd be concerned about infection. Women do sometimes get infection with IUD. I'd not guess pregnancy as much. They could have swabbed the mucus and examined it. Also remember that IUD's have hormones involved often and that also can cause excess mucus for some women. anyway, what did your doctor say? If you didn't ask at the time, I think you should be able to call and ask this question!
Did the doctor have you do a pregnancy test after he came up with the mucus plug?