Of course you should not be worried. The sensitivity of pregnancy tests is nearly 98%. However I blood could be investigated to measure HCG level (Human chorionic gonadotropin
, 0-10 - normal for non pregnant). You may need your gastrointestinal tract to be examined precisely but it would be also a good idea go to psychologist
to talk about your fears. Your hormonal panel (FSH, LH, progesteron, prolactin, estrogen, etc.) should be also inspected to reveal some hormonal distubances as a course of irregular periods
should be also done (to reveal possible ovarian cysts).
Wish you Good Health
Dr O. Aleksandrov