2 weeks after intercourse period came, but different from usual. Bright red blood and only lasted for 3 days. Could it be a pregnancy or miscarriage?

It's possible. It's possible. Do a pregnancy test to find out.
Take the test. If you want to know if you are pregnant, take a test. In today's world you do not need to wonder if you are pregnant or not based on symptoms. Pregnancy tests are available at any pharmacy and in minutes you will have your answer. They can detect a pregnancy 7-10 days after conception. The best time to take one is when you miss your period.
Depends. ? previous period's date ? If bleeding is 5-6 weeks out the previous one then pregnancy and its complications are possible. Get a preg test done immediately. Other possibility is you were already pregnant at the time of intercourse. Now is spotting . It all depends on when was last mens and if you used and protection. Again, start with a simple preg test. You may need pelvic exam.