Could I be pregnant?

I got implanon/explanon inserted in February of 2017. My boyfriend of 2 years and I have unprotected sex frequently. Lately my stomach has been growing as if I am pregnant, the same way it grew when I was pregnant with my son . I’ve been nauseous, I have increased appetite as well as increased urination. I’m aware the implant is supposed to be good for 3 years. I’ve taken a few home pregnancy test that have come back negative. Could it be just typical weight gain ? I plan to seek advice from a doctor but has anyone gotten pregnant on this birth control? If so how’d you find out?

*edit: I also haven’t had a cycle since September of last year*