Nexplanon periods?

Lillian • 19 years old ~ Married ~FTM to identical twin boys ~

My twin boys are officially 4 months old. I got the nexplanon implant at my 6 week postpartum visit. Since then i had been spotting non-stop up until about they were 2 and a half months old, I think? And then all of the bleeding just stopped. I was used to the bleeding going away for a few days and then returning because there would be times where the bleeding would stop for a week and then come back for three more and repeat. But now my bleeding is just gone. Should I be worried? I kind of feel like I could be pregnant, but all the tests show negative and I have low iron and low thyroid. Thoughts?