Hi, A.. I don't know much about the mirena, but I know that you will, indeed, bleed for a while after giving birth, and that's normal unless it's very heavy for a very long time or smells bad. Bleeding for 4 weeks, which gets progressively lighter and then goes away, is normal after giving birth.
You will not get your period back right away. It may take a few weeks or a few months. You are very smart to start using birth control right away so that you don't risk getting pregnant too soon.
You're supposed to wait 4-6 weeks after giving birth before having sex. That's to give your body time to heal and stop bleeding. It's very, very UNlikely that you could get pregnant 4-6 weeks after giving birth, so this is something you should not sit around and worry about.
If you're breastfeeding, you probably won't get your period again for at least a few months. If you're not breastfeeding, it will probably come back sooner. Even though you don't have your period, you still have a chance of getting pregnant -- again, very smart for you to use the mirena just in case, even though your period hasn't started up again.
Enjoy your baby, and don't let yourself fall into worry. It sounds like you are doing the right things.
Peace,
Syl