I just had a baby 6 weeks ago. Yesterday I had my 6 week check up and got the mirena iud, last night my SO and I had unprotected sex but he pulled out. My question is, how effective Is the mirena the day you got it placed? And should I buy the plan B pill? What's the chances of me getting pregnant if I were to buy the plan b pill and use it within 24 hours

IUD are almost instantly effective so I don't believe you need Plan B. There is always a chance, regardless of the birthcontrol method, that you will become pregnant. Watch for your next period. After just having a child it might be irregular so don't expect it on time.