Hello, I would be happy to help you with your question.
3 weeks after unprotected intercourse
, if you had conceived, it would not be too early for a pregnancy
test to turn positive. While these could be cycle related symptoms that occurred after ovulation (sometimes there is a small amount of spotting mid cycle with ovulation) I think that a pregnancy test (either urine or blood) is a great idea!
I hope that this helps and good luck!