Is Missed Periods While Having Endometriosis A Cause For Concern?

hiii,i am 35 yr old,diagnosed as severe endometriosis..usually ve polymenorrhoea and severe dysmenorrhoea,my cycles were regular 28 dys for past 2 months,but my period is missing this month,lmp was 5thfeb 15,on 11th march i did a urine pregnancy test which came neg,,what shud i do now?