This is not normal and should be addressed. It could be that it started to have its expected effect of lighter cycles, but now is "misbehaving" for the following possible reasons:
1. It was not placed properly and is in a funny position
2. It has migrated down into the cervix
3. There are polyps or fibroids present that are now bleeding related to the IUD
You need a visit and an ultrasound
. If this does not solve things, you might need it pulled and a followup ultrasound called a "sonohysterogram" to look for polyps. Good luck!!