Period ended on July 27th. Had unprotected sex the next day. Three weeks later I lightly bled for 6 days. Am I pregnant?

Not likely, but you. should do a home pregnancy test if you have any concern. Use first morning urine and follow the instructions carefully. If the test is negative it would be prudent to repeat it a week later to make sure.
Unlikely. It does not sound likely but if you are worried you could do a pregnancy test to be sure. If you are not trying to get pregnant, you need to be using a reliable method of contraception and condoms to decrease the risk of sexually transmitted diseases.