Is my IUD starting to loose effectiveness?

So I’ve got a Mirena IUD and it’s never let me down and helped massively with some health issues I had prior to it being put in. Had it 4yrs and my last period was the most painful I’ve had since it was put in - dizziness, cramps, back pain etc.

I’ve been a little stressed lately and my cycle was weird - spotting for 3 days and then a full week later my period hit (was so paranoid, I actually took a pregnancy test the day before I got my period and it was negative) . Now I’ve had this weird stop/start cycle twice last year and I’m wondering if it’s just stress related or if maybe the pain and everything suddenly coming back is a sign it’s starting to loose effectiveness? Especially given it’s basically in its last twelve months. It my first IUD so I’m curious if anyone else has had similar experiences 🙂