Uterine lining shed ?? (a bit tmi)

Hey girls!! i’m 16 and this is my first time on birth control and i just got the mirena on thursday and it is now saturday. i was using the bathroom and i felt a huge chunk of uterine lining come out and i looked down and it was like half the size of my palm probably and i’m just a bit confused as to what this is from and a bit frightened. it wasn’t super bloody just a brownish color similar to my spotting. i was just wondering if this is normal or what caused this. my doctors office is closed on the weekends so i thought i’d ask here first. i know that the uterine lining is supposed to build around the mirena so should i be concerned from shedding so much? has anyone else experienced this? i also ended my period like 4 or 5 days before getting the iud so it’s not my cycle i don’t think.. idk please share some knowledge and help if possible :,)