I would call the doctor's nurse tomorrow and let them know what is going on. If you take plan b let them know that too. They will be the best ones to guide you. Don't be embarrassed, lots of ladies have walked tge same path your on and it's not new to them. The most important thing is to get you help.
You can definitely get pregnant now!!!!! Just ask Tori Spelling.
If your not breastfeeding go get a morning after pill and take it ASAP. If your breastfeeding, call the doctor, nurse or ask the pharmacist if it's safe to take while breastfeeding. I believe plan b is safe, not 100% positive. As far as the Mirena you will need to tell your doctor about the unprotected sex and they will either 1) have you wait to get it until your next period or 2) do a blood test to see if you are pregnant again. As long as your not pregnant they can place it, but they need to be sure your not. They will most likely do this even if you take the morning after pill. Don't have sex again or use a condom until you get your IUD placed. Good luck