Hi, I Had A Pap Scan On August 19th, And

Hi, I had a Pap Scan on August 19th, and she told me to come in for a ultrasound September 7th because my uterus is on my left side of my body. I had sex Saturday august 13th and wasn't sure if she could of notice if I was or wasn't pregnant, and today I was suppose to get my period but instead ive been having dark/light brown discharge and I'm not sure if I can be pregnant.
so my questions are, can u detect pregnancy from a pap scan early?
why hasn't my period come today and only have light/dark brown discharge ?