As she gets her period, then there is no chance of pregnancy
.Pregnancy is possible only,if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period.
She is probably suffering from irregular period and the possible reasons are hormonal
imbalance, wt gain/extreme loss, stress/ anxiety, thyroid
problem, irregular eating habits etc.
Need to investigate properly to rule out underlying pathology by USG lower abdomen,Thyroid profile,LH/FSH etc.So consult with your doctor and take suggestive measures.
Avoid stress,take healthy diet with iron supplements
,control body wt, proper sleeping and practice safe sex in future.