Please check with your breast specialist like Bluebutterfly suggested.I have added some information you might want to discuss with your doctor since Minera IUD does release hormones.
Women who currently have or have had breast cancer, or have a suspicion of breast cancer, should not use HORMONAL contraception because breast cancer is a hormone-sensitive tumor.
Women, who do not wish to use a hormonal method of contraception, can choose a copper containing intrauterine device that is as effective as the one releasing hormones.If you are using the copper one then you don't have to worry about hormones being released in your body,but still discuss it with your doctor.Most of all continue to have this lump investigate until you know exactly what's going on.Best wishes.
From your previous posts it sounds as though you plan to continue seeking a dx in regard to your lump and itchy patches. If you do have further studies and see a breast specialist, your question about Mirena would be a good one to address with him/her.