Really confused and worried

Me and my partner have been having unprotected sex for 3-4months but i havnt got pregnant is this normal? I had the nexplanon implant taken out in january 2016. Just concerned as i thought there could be something wrong with us. I have had strange periods though, being quite late and i missed a period in november but bfn test. Atm im 4 days late but not worried much as i was 2 weeks late last month but af finally arrived. Any advice would be great just to rest my mind thankyou xx