C.B.
I have a Mirena (progesterone secreting) IUD in and love it. I had one after my second child also (the current one is my 2nd IUD, 3rd child). I have heard of reports from women who have cramping/bleeding after insertion, but it usually subsides by this time. I have never had a period while having an IUD in place.
I would recommend you contact the MD who inserted it to 1. verify correct placement; 2 rule out problems (which due to liability issues I won't go into); and 3 discuss a timeline as to how long to give the symptoms to resolve vs when to decide to have it taken out. If there are no other problems, it is just a matter of your tolerance level. It also could be simply that the string is too long and irritating the surrounding tissue.
In general, the IUD is a very good form of birth control for women who have given birth before and are in a monogamous relationship.
Good luck!
C.