Please help!! Is it my period ?

My period is supposed to come in 12 days and then about 4 days ago I went to the bathroom and my underwear was stained. I thought my period came early but the next day I didn’t bleed anything, I’ve been having brown discharge for the past 4 days?? It’s a very small amount and if it’s a lot then it only happens once a day. I don’t know if this normal? Is this my period? My boobs have been sore and I’ve also been having small cramps every now in them. I did take a plan b pill a week ago, could this be a side effect?