its long but can someone give me advice😒

So i've had the nexplanon birth control only for 7 months . I got it removed september 1st due to it never stopping my cycles. I had a AF every single month while i was on it.My last AF was august 24th according to glow my next AF was suppose to be sept 21st still nothing yet it is now sept 27& still no AF. i have been BD since i got it removed & had sex a few times during my fertile window . I took a few hpt but getting BFNs ☹️ . I also been having really bad lower back pains. Like i did with my 1st child . I just dont understand , i never missed my menstrual even while on nexplanon i always had one.Can i be pregnant and getting false negatives?