Does anyone have experience with the Mirena (5y IUD) starting to become less effective after a few years?

My son was born Mar 2015. I had my first IUD placed Jan 2016. The cramps, spotting, weight gain, etc all leveled out after ~6 months and it's been smooth sailing at every checkup. Normally during my "period," I go through mild PMS symptoms, e.g. appetite changes, light cramps, tender breasts. There's a 50/50 shot I spot lightly, and it's hardly ever enough to even need a pantyliner.

Fast forward to this cycle at 4.5 years in, and I'm on day 10 of bleeding. The first 4-5 days were normal with minimal spotting, then a gap day (not unusual), and more spotting which is becoming heavier rather than lighter. Today I bled through a regular tampon in 3 hours. It's been years since my flow was heavy enough for that. There's no crazy pains (just my normal amount of cramping), the pregnancy tests I keep just in case were negative, and it feels like a normal cruddy period would (instead of a normal IUD period- hope that makes sense!).

Are the hormones wearing out? Is this normal towards the end of the IUD's shelf life?

TIA!