How To Get Normal Periods?

Hi, I am currently taking cerazette (POP) and since i started i've had no periods or spotting whatsoever. i recently missed two pills in a row and i had also missed one pill earlier in the pack. I am sexually active and cerazette is the only form of contraception i use so i'm wondering what the chances are of falling pregnant? Also, if the chances are quite high, when is a good time to take a pregnancy test? Because i dont have periods anymore i dont know when i would normally ovulate or normally get a period. Any help or advice would be great! Thanks much!