Positive pregnancy test the day after intercourse? (went for blood pregnant test 2 days later. Hcg levels were 600ml/iu).

Sounds like. You must've gotten pregnant before that.
Previous intercourse. It is not really possible for you to have a positive pregnancy test due to intercourse and impregnation the day prior to the test; more time is needed for the fertilized egg to implant and start producing the hormone necessary to make the pregnancy test positive. It is more likely the positive test is due to impregnation from previous intercourse.

Related Questions

Positive pregnancy test the day after intercourse then hCG levels were 600ml/iu 2 days later? Pregnancy this soon?

Yes. Although the pregnancy is more likely due to a different episode of intercourse than the one the day before taking the test. Alternatively, considerably much less likely, you could have another source of HCG produced in response to another (non-pregnancy) medical condition. Read more...