Help please

I was once on depo back in March it was the only one shot.. Then I finally got my cycle on September the 28 and I had sex on October 7th, 12th, and then I had a slight spotting on the 16& 17 with a little cramping.. Did a home pregnancy test on the 18 and it was negative. Then had sex on the 19th and 26.. My cycle was suppose to come on the 26 but it still hasn't came