What Causes Stoppage In Periods?

My fiance takes the contraceptive pill "Yasmin" on a regular basis. She had not been taking her pill for a week or two before the last weekend in January at which time we had intercourse, immediately following which she took the emergency contraceptive pill "plan B one-step". Two days later a similar situation occurred and she took a generic version of the emergency contraceptive (on Sunday the 31st). She was supposed to receive her period on or around the 8th and has not received it to date though she has halted all contraceptives. She took a pregnancy test yesterday and came back negative. Now we are left wondering if we should go back on the pill later this week in the event that she does not get her period or if we should wait until her body corrects itself.