Frustrated

So I had sex on my most fertile day and the day after. A few days after I started getting sore breast and lower back pain ever since. Recently I volunteer to let a sonographer student practice a pelvic ultrasound on me and she indicated to me that my lining looked thick and that my periods is coming soon. This was a week ago sadly today am bleeding lightly. Can I still be pregnant or is it really the start of my periods????