It's possible you could be, but like pp said, plan B changes your period.
As it is not a form of birth control and you do not want to get pregnant, id advise you to get on birth control and use condoms...
Its a lot easier than worrying about being pregnant. And more responsible.
I would take a home pregnancy test and go from there. Best of luck to you.
If u have any doubts a simple pregnancy test can help... although from your history pregnancy seems unlikely but u should use a regular and reliable contraception if u r not ready for a pregnancy as your current practice is not so safe for regular use.
I would check just to make sure as you didnt take it within the requied time frame and this can make it ineffective, I have fallen pregnant on plan b and took it as I should so I would do a test, they only count spotting as a period if it continues ofr 3 days straight, as for feeling not ready to have another baby, I have had alot of my babies close yogether and honestly the majority wernt planned that way, it may feel overwhelming at first if you do find out that you are, but dont rush into anything,, my children that are close together get on alot better than the ones with a bigger gap inbetween :)
Don't worry!! Plan B changes your period. The brown discharge means it worked. I had one in August and my period changed for a couple of months I was always late a week. You should be fine