Hi and thanks for the query,
By no way can exclusive oral sex
lead to a pregnancy
. Its relationship to menstrual cycle abnormalities is really very obscure.
However, the commonest cause of lack of periods or delayed periods in a female of reproductive age, with a past history of risky or unprotected sex is a pregnancy. The most appropriate thing to do now is carrying out a good pregnancy test to rule out pregnancy. Its only on the basis of a negative pregnancy test that other causes of menstrual abnormalities could be considered with obviously expert opinion from a gynecologist.
For now, I suggest you carry out a pregnancy test first.
Thanks and best regards,