Amanda - I can't remember where you are in your menopause? Are you technically still peri? It sounds as though you are still producing some of your own oestrogen so you will be getting times when you have higher oestrogen. If you are peri meno and taking oestrogen then the problematic bleeding may indicate that your womb lining is building up too much because your own hormones are kicking in. Your tender breasts would also be a sign that your own cycle is still happening occasionally. The peri stage can be very tough.
Can you still feel the strings of the Mirena? - sometimes the Mirena drops out!
One of the plusses of the Mirena is that it should reduce and usually eliminate all bleeding.
I'd see your GP for a check up. DG x