Well, if you had unprotected sex around ovulation, that is the most fertile time in your cycle, and since you had unprotected sex more than 1 time around ovulation, that ups the odds. Did you have unprotected sex other than this before you ovulated? Sperm can survive up to 5 days if conditions are right. Is possible that you ovulated earlier than you thought and had unprotected sex then, if you were just going by a calendar and not basal body temping and checking your cervical mucus?
The strangely sore breasts with the nipple sensation sounds a lot like it could be pregnancy related--perhaps you had early implantation. This usually occurs at 7-10 days past ovulation, but I have read that rarely it can happen at 4-5 DPO.
All you can do is wait and take a test. Some of the early response tests claim they can detect pregnancy 5 days sooner (it's really more like 4 days since they fudge a little on the box with the 5th day being the day your period is due). If you are eager to know and can't wait, buy an early response type test (like First Response Early Result or Answer Early), get a box with 2 or 3 tests, and then test 4 days before your period, on April 7. If that test is negative, you can test again on the 9th and then again on the 11th.
As far as telling your boyfriend, that's your decision. If you are pregnant, he will need to know, obviously, but as for now, you're kind of just in limbo while you wait.