Need help with a question regarding pregnancy

First off I'm a male and a major idiot as I didn't use a condom when I had sex. I know, my fault and fully aware of stupid it was of me. Anyways, less than 24 hours after sex her period began.

So the Question: What are the odds of her getting pregnant? And should i still get her the plan B?

other details: she is 45, im 28. Not sure how much age plays a factor but adding that detail anyways.