What Causes Prolonged Vaginal Bleeding While On IUD?

I am 36 years old, 2 kids, and had current IUD for 2 years. I have bleeding for at least half of the month. My last PAP 15 months ago was fine. Should I see my gyno or is this fairly normal. And my first IUD after my first child, my period was regular after about 6 months. Thank you