Had sex 4/3 took plan b 4/5 had unprotected sex 4/10 found out i'm pregnant 4/17 did the plan b work? or this is due to the 2nd sexual encounter ?

Not possible to tell: Depending on your HcG levels and the sensitivity of the test it is possible to get a positive test 7 days after conception. So, with that fact, it is impossible to tell from the information you have given which encounter led to the pregnancy. An early U/S MAY give a better "guess" but this is something you may never know. If male partner was different - then paternity testing would be advised.