Hi JellyB
I had mirena coil fitted for contraception about 7 years ago. I put on weight and breasts were tender then settled down. It proved good for me overall and I had it in for around 4 years.
I recently had another one fitted after D&C, as part of HRT. I have not had any breast tenderness this time, and the small amount of weight gain is most probably related to festive season and eating naughty things right now if I am honest with myself!
I am post menopausal, age 55 and have not added the oestrogen part of the HRT yet. It seems to be fine so far (since August 2011). I am open minded and would add oestrogen if required, or have mirena removed if it becomes a problem. (I should say I had a year of conti HRT prior to the operation and mirena).
It can take a little while to settle down but it may prove to suit you, so give it a little time and I hope it works for you.
Fx