Can Contraceptives Disturb Periods?

hi.. I had unprotected sex on may 24 2010 around 2:00am and took the plan b pill at 3:15 am.. and then started spotting/ bleeding on may may 29 2010 at about 3:30 am is this the cause of the plan b pill...or should i be worried.. i usallly start my cycle at the begining of the month or at the end.. i have already had my period at the beginning of may.. soo this is why im concerned