Some women can be pregnant and get their periods. This is kind of rare though.
The only way I know of to find out if this was a miscarriage would be to see your Dr for a blood test. If you were pregnant the hormones would show up in your blood.
This happened about 3 weeks ago, right? If so, your hormone levels would be back to normal by now so a blood test would not help at this time.
The other thing I was thinking this might be is implantation bleeding.Not all women get this though and if they do it's usually very light bleeding. It happens within a week of unprotected intercourse, if it's going to happen.
When are you do for your period? If it's late or seems unusual take a pregnancy test.
I wish I could have answered your question but it's too late to know if it was a miscarriage or something else going on.
Your post really helped inform me as to what could have been going on, thank you so much