I'm assuming you mean a morning after pill. They have high levels of hormones, and can mess with your period. Because of the odd timing, even a first response test *might* not be accurate for another month, although if it's positive, that's probably a true response. You should probably consult with your ob/gyn.
For the future, you need a different form of birth control. The morning after pill is not meant to be your primary method of birth control.