Hi,
If you have regular 28 day cycles, then your ovulation should occur 15 days before the next expected day of period. This comes out to be 13 days after your first day of period of previous cycle. It is unlikely that at the 6th day after period, you should ovulate. It might be due to ovulation that you are having symptoms of pain and bowel upset or it might even be a mild stomach bug. In any case if you want to confirm about your pregnancy, then you can use a home pregnancy detection kit, but a pregnancy is ulikely if you have regular 28 day cycles.
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regards
Dr. Gupta