Menstrual Clots

So i’ve never had menstrual clots before without birth control and last month i got on nexplanon and ever since then everything has been fine i’ve just had brown discharge. I just got my period this week and it’s been very light, but the other day I got a menstrual clot as big as two bottle caps and it was thick. I’ve never had a menstrual clot before until nexplanon so idk if i should be concerned or not. I just wanna know why that happened, if anyone can help.