You are just paranoid
There is no chance you can be pregnant. But the side effects from Plan B will likely make you panicky and confuse you, so be warned that it is not pregnancy, but the Plan B messing with your cycle. For example, you may get withdrawal bleeding about 7 days after taking it. Your real menstruation may be delayed or absent, and your cycle may change dates and length a bit for the next few months.
You are right to keep your underwear on. At least one of you should always have underwear on. It is also good to end with his ejaculation and then clean up, rather than have him go first and start to worry.later about touching him and transferring semen into your vagina.
Best of luck, and remember to stay safe!