However if you were having really heavy periods up until February when you were either 57 or 58 - this in itself is unusual, more than the bleeding after fitting the Mirena. More to the point did you have the heavy bleeding fully checked out before you had the Mirena fitted, to rule out any pathological reason for the bleeding - eg fibroids, polyps, hyperplasia etc, rather than just very late menopause?
If everything was OK prior to starting the Mirena it is hard to say what the reason is for the bleeding without getting checked out so at your age I would definitely go to the doc to see whether in fact the lining has now thinned too much as Tinkerbell suggest (as you have had progestogen only for a long time). If you were having regular periods up until fitting the Mirnea then your own oestrogen should not have dropped very low as yet although I imagine it is still possible for overthinning to occur.
An indication of whether your oestrogen has dropped would be if you are getting hot flushes, night sweats etc, but if you are taking other medication eg ADs etc and some others - then you may not experience these so you wouldn't know. As Dancinggirl says replacement oestrogen may well be helpful at this point!
Defo go to the doc - and if necessary ask for a referral to a specialist!
Hurdity x