BASED from the information provided you had a sex but used protection and that too one month ago put you in the REALM OF "NO- PREGNANCY"
Here are things that can lead to delay the menstrual cycle.
2) EXCESS OF COFFEE,
3) Significant weight gain or loss
4) Excessive alcohol use / SMOKING
5) Poor nutrition (or a diet too high in carbohydrates)
Let explain about ovulation and pregnancy
The process know as "OVULATION" is when the female egg releases from OVARY, this occurs roughly between the 14 th -19 th day after the menstrual cycle (end of bleeding ), pregnancy means the meet in of the man's SPERM to the female's OVA- both have half genes which need to get together in order to form a complete structure, so the best possibility for the pregnancy is if the intercourse has occurred during those five days, other time their is no OVA present to meet the sperms hence no possibility of getting pregnant
1) IF you don't get period with in next 5 day , Just to be on safe side please take URINE PREGNANCY KIT TEST- Its just stick where you have to urinate on that and its available over the counter and not doctor consultation in needed for doing the test, just follow the direction in test kit.
2) Being over weight do NOT STOP PERIOD - but still being obese
in not good for health so please try to cut down food , or replace some with salads and do exercise.
3)May need a CONSULT FROM OB/GYN in case repeated irregular cycles.
Hope it helps