You are right. The first thing that needs to be done is to rule out a pregnancy. A pregnancy test may give a false result if it is taken too early. Hence repeat the test after 8 days or better still go in for serum beta HCG levels for confirmation of a pregnancy. Another cause can be hormonal changes like hyper-prolactinemia. Further evaluation will be necessary. Consult a gynaecologist for an evaluation.
Hope this helps.