Is it true that plan b won't work if i was already ovulating when i had sex ?

Not quite true: Plan B works through different mechanisms: It prevents ovulation, makes it more difficult for sperm to get to the egg, and makes it more difficult for the fertilized egg to implant. Plan B is most effective when ovulation has not occurred, yet - after that it's effectiveness drops from over 95 % to 89 % effective. Bottomline, a woman wants to take it STAT.