nipples can be sore if they've been injured (have you been especially rough with them at all?). overall breast soreness is usually hormonal (unless you've been beating up on them, too) with swelling and tenderness peaking before your period.
If you are concerned about pregnancy, either wait till you have missed a day of your period, or 10-14 days following the sexual encounter in question. then take a pregnancy test using the first urine of the morning (it's most concentrated).
If it's negative, count your lucky stars, drag you and your boyfriend off for STDs screens and onthe way home, buy a big box of condoms. then promise each other that you'll always use them, and use them correctly!
If the test is positive, then you've got a lot of big decisions to think over, and the sooner your decide, the better.
Summertime, and the living's easy...