Hello,
Welcome to ehealthforum and thanks for writing
First of all relax yourself as chances of pregnancy are remote.Plan B is an emergency contraceptive pill and is not recommended for regular use as it is high dose hormonal pill and causes alteration in the normal hormonal levels leading to altered cycles.
It seems you have messed up your cycle by taking Plan B within a short period of time consecutively.
It can also cause delayed next period by a week or so.The periods might start soon and do not stress as it can further delay the cycles.
Consider using regular form of contraceptive as pills,deposhots etc for prevention from pregnancy.Wait for a week and if the periods do not start then get a Urine test done for pregnancy determination.
Take care!