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You had the period, even for one day means there are 99% no chances of pregnancy
. but you need to check out your past history of periods duration, if you never had any such short period then just go for a Urine pregnancy test in morning. that is very easy to do and test available at stores. in very rare case inspite of menses, pregnancy continues. just relax and do the test.
I hope I answered your query. Wish you good health.