Swollen breasts three days after sexual intercourse.

Tender breasts are breasts that feel slightly more sensitive to the touch, sometimes to the point of being painful. Swollen/sore breasts are a pregnancy symptom, but they are also just a sign of PMS. I'm sure you have experienced that before, that your breasts are just slightly bigger and less comfortable in the days before your period? (If you haven't and this is very unusual for you, I'd say that makes it more likely that you are pregnant.)

You can always speculate before you either get your period or miss it, but the fact is that you can't know for certain if you're pregnant until you KNOW, either because you get your period and you're not pregnant, or because you missed your period and then got a positive pregnancy test. 

If you're not pregnant and you don't want to be pregnant, next time use condoms or go on the pill. If you happen to have unprotected sex, well, it's always good to stock up on the morning after pill.