It is not possible to get pregnant in your case if you have sex a day prior to the onset of your menses as the endometrium had shed off and as there is no endometrium now, so there would be no implantation even if there is fertilization in cases of delayed ovulation.
So, rest assured, you are safe.
I hope i have answered your query, wishing you safe sexual practices,
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.