A week early?

Not sure if it's normal but my period came a week early! Is that normal? Lately my period has been acting up! Me and my fiancé have been trying for about 6 months to conceive, I've also have been taking prenatal vitimians! But nothing! I have a 7 year old daughter and 3 year old son from my previous relationship! I'm not sure what's going on but people tel me I should take a plan b to clear my system? I haven't tried because it sounds silly! I've scheduled an appointment with my OBGYN for the 11th to see what's going on but I thought I'll ask around to see if anyone else is going through this or has gone through this!