Please help, what is this, tmi

I'm not sure what this is. My period was a while late and I'm sure it was because I started birth control (nexplanon). I've now been on BC for three weeks and i started my period a few days ago. At first it was very light and mostly old blood, but today I go to change my pad and this is what I saw. Please help a girl out!!

Could it just be uterine lining?? It wasnt really bloody and it had the consistency of gum almost.