So, you would not have pregnancy symptoms or know you were pregnant three days post unprotected sex. Keep that in mind. And a pregnancy test would not be accurate at all. The earliest you can get an accurate pregnancy test result from a home test is 2.5 weeks after the unprotected sex. You may be having an off period. This does happen. It can happen out of nowhere. A hormonal shift can happen at any time and wham, our period is funky from how it normally is. Implantation bleeding from my understanding is just a tinge of blood. Some women do spot when they expect their periods but it's hard to say with the timing of things. You very well could be overthinking and remember, even our stress can affect our period. So, I'd try to stay distracted and when 2.5 weeks comes about, take a test. Unprotected sex obviously warrants a test if you are late or things are weird but you have to time it right. And shouldn't worry until then. Let me know how it goes and what the result is.