IUD too good to be true ?

Hey so I got my Mirena inserted on the 15th of December and my period started on the 14th. After the insertion I had mild pressure like cramps that lasted only for the evening of the insertion. Since then my period has been a lot less and by now I'm just spotting which is odd for me cause I usually bleed a good amount for 7 days. My boyfriend and I had unprotected sex on Saturday (a little under 48 hours after the procedure) but since I was on my period when it was inserted, it says that the Mirena was effective within 24 hours. Just looking for a reassurance and that all of this is normal and that my decision wasn't stupid.