I had sex two days before starting what i think was my period(july16) it was slightly shorter. i have swollen breasts now it's august 7.am i prego?

Probably not: You are probably about to have your next period, although it is a week off if I did the math right. The only way to be sure is see if you miss your period and then check a pregnancy test.

Probably not: however if you miss your period you need to do pregnancy test.

Possibly: If you've had sexual intercourse with penetration (protected or not) there is a risk of pregnancy. My suggestion is to take a pregnancy test - which should help you to determine if you are pregnant. Typically a test will be positive approximately 2 weeks after sexual intercourse if you are pregnant. Unfortunately, your breasts can become tender both in pregnancy and during your menstrual cycle.