Plan B during Ovulation ?

The only pill that is proven can work to prevent pregnancy during the 24 hours around ovulation itself, is Ella (Ulipristal Acetate). Plan B (Levonorgestrel) might work, but it has not been proven.

Whether you were in your actual hours of ovulation, may be difficult to know. If you were relying on an app or website to estimate your cycle, then they are, indeed, just estimates. The only ways you'd know how close you were, is by charting your fertility symptoms each day, in advance, or by using an ovulation detection kit.

You might want to see about getting a Paragard IUD fitted by a doctor within 5 days/120 hours of the sex, which may prevent implantation of the pregnancy, if the Plan B does not help, given that you think you were ovulating. Otherwise, if you were fertile, then there is definitely a possibility of pregnancy. The only way Plan B is proven to work, is by delaying ovulation. If it was too late to do that, then it may not be able to do anything (it's possible it may work in other ways, but it's never been proven).