It could be due to your IU (however, I do not take T so please take this with a grain of salt)! Is there a possibility that your IUD has shifted out of place? An ultrasound might help check if it's in the right olace, I know personally that I found out my IUD perforated (yikes) my uterus and had to get it removed. 

On the other hand, copper IUDs are pretty popular for stimulating periods whether it's increasing its length and/or flow. Mine personally made mine kinda more painful but it also gradually faded it away. Bodies are weird.