What Causes Extreme Back Pain,cramping And Light Spotting?

Hello I decided to get on a birth control little over a year after having my son. I got the nexplonon implant February 19, 2014. I was on my period same day I got this, I bled all the way up until end of July 2014-beginning of august 2014. Well august 4-5 I had unprotected sex with my boyfriend, he didn t pull out. I never actually considered pregnancy was possible I have birth control, it never failed me before(my son was planned), well I did some research today and more people than I thought get pregnant while on this. Well my concern is I could be? On august 14th, 2014 I was put on an antibiotic, I finished it up the following Sunday, I was only on it 4 days. I m aware antibiotics cancel out birth control. Well recently, like past week I have been getting extreme back pain and horrible cramping! I ve been bleeding/spotting and one day it was heavier then spotting, other than that no period no bleeding nothing! So the question is, is it possible I could be pregnant? Even if I didn t begin taking the antibiotic a week after sex?