What Causes Watery Bleeding During Periods?

Hello, I'm 40 years old, 167cm height, 48kg and I'm using IUCD, lately while my mensus, its always watery blood. Not like regular thick blood. I am using this IUCD for the pass 3 years and I suppose to take it out last May and I still not doing it yet. Does it because of what I'm using or any another thing that relate to it. Thank you.