Brown discharge after Plan B?

Those are very common side effects of Plan B.
From the dates you've given, you may have been very close to ovulation.
If you felt you needed emergency contraception at that time, I would have advised using the other emergency pill, called Ella (Ulipristal Acetate), as that is the only pill that is proven can work in the 24 hours around ovulation (& even that pill isn't guaranteed). Plan B might work at that time in your cycle, but it has not been proven.
However, as the condom did not split, & assuming you were careful not to get his sperm or precum around your vaginal mucus, then you should be fine.
You may get other hormonal side effects, & you next period may be late or early, if not on time. Plan B is a huge dose of artificial hormone for your body to deal with. It can take some women months to get regular periods back. Usually things are just out of sync for a month or two, though.