Thank you for your post! I'm sorry to hear that your IUD has implanted itself onto your uterus. As long as you are not having adverse effects, then you're ok to wait until the 28th. You're having pain because of the location of the IUD, but not necessarily because it is causing more harm. However, if you're having severe pain related to the IUD, then you may want to call your doctors office and inform them about it. Maybe if you insist that the pain is unbearable, they may move up your date. You can also ask for them to call you if there is an opening sooner than the 28th.
Best wishes and keep up posted,