13 day period

I took Plan B on January 2nd. I anxiously awaited my next period, which was projected to come on January 15th according to multiple period tracking calanders. My period came early on January 10th. Today is January 23rd and I am still on my period. I understand Plan B can alter your period by making it heavier or lighter or shorter or longer, but my period is usually 7 days and this is just concerning. Any ideas?