@curiouscat No. You aren’t pregnant until it implants. Plan B/the morning after pill prevents fertilization and may prevent implantation. You are absolutely not pregnant when it does its thing. Beyond that, all indications show that if you are preggers and ingest some EC on accident, you’re going to stay preggers.
Spermies do live for 72 hours, but they also have a swim ahead of them. EC works best when used immediately because there is less of a chance the sperm have already reached the egg. That is why it decreases in effectiveness the longer you wait—there is a greater chance that you already got fertilized and they aren’t sure on the efficacy of it preventing implantation.
As for bleeding—it’s probably just withdrawal bleeding. From the wikipedia article: Temporary disruption of the menstrual cycle is also commonly experienced. If taken before ovulation, the high doses of progestogen in levonorgestrel treatments may induce progestogen withdrawal bleeding a few days after the pills are taken. One study found that about half of women who used levonorgestrel ECPs experienced bleeding within 7 days of taking the pills. If levonorgestrel is taken after ovulation, it may increase the length of the luteal phase, thus delaying menstruation by a few days.
So the short answer is that it looks like there is no reason to be concerned, but if your girlfriend is still uneasy about it, she should absolutely contact a Planned Parenthood specialist. Her health is nothing to play around with.