Hi,
Welcome to the forum and I am glad to help you out. The gastrointestinal problem can interfere in the absorption of the drug resulting in the failure of the oral contraceptive.
Ideally the urine for pregnancy test should be done 1 week after missing your periods. But you can test at 9 or 10 days past ovulation with a sensitive ( 20 - 25 mIU ) pregnancy test. Aimstrip pregnancy test gives good results and can detect 20 mIU of HCG.I sincerely suggest you to do a urine for pregnancy test and if the test comes negative, then get the other causes of delayed periods ruled out from a gynecologist.
It can happen because of hormonal imbalance, increased stress, eating disorder, increased smoking, alcohol or caffeine use and medical conditions like PCOS, fibroid, polyps and endometriosis. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.