If not a blood clot , then what is it ??

Today I received my period after being about a week late. This is my first period with Nexplanon. My period started out slow , but came to a medium flow today. Earlier this afternoon I came to a surprise when I saw a blood-clot looking substance with my period blood ( I apologize if TMI ) it seemed to be mucousy and a bit firm. It seemed like a colorless membrane. I can't quite say much about the details of it. I was at a friends house so looking into what it was , was not an option for me at the time. Please let me know if anyone has experienced this with Nexplanon or birth control in general.