What Causes Missed Periods Inspite Of Being On Nexplanon?

Hi I'm 20 years old and I've got a 17 month old baby I got the implant Nexplanon 68mg I'm 2 months late I was on antibiotics for a urine infection and me n my partner sleep together and now I'm getting the same syptoms as I had with my little girl. I have done a pregnancy test it came back negative but could I be pregnant.