Hi, I'm 18. I Had Sexual Intercourse On The Date

Hi, I'm 18. I had sexual intercourse on the date of Nov 6, 2015. Now let me explain about my PMS. I used to have irregular periods. But during the last half years of 2015 my periods became regular monthly normal periods. So anyways, after I had intercourse, I had my period on the 19th of November. My period was heavy. (yay I'm not prego!) BUT! I took a test just to double check, and i happened to have taken it on a day i was in a rush. I was running late for work when i decided to take the test. I waited 5-10 minutes in the restroom. It came out a faint positive, being as i was in a rush. I threw it away. And forgot about it. Then December came around. My period became much lighter. Light brownish pinkish depending. Instead of 7 days of menstrual cycle I was shorten to 5 or so. During one of those days it wasn't too light nor too heavy. And now its January. I missed my period. I'm about to test after the snow has melted more and when its more safe to drive on the grounds. But my breasts are actually super tender. I never take my bras off to bed, but nowadays I have to Bc it hurts that much. And also my lower back near the waist or so is aching a lot. I've never had back pain like this. Please tell me if I could or couldn't be pregnant.