I don’t know your full history but I can give you some general information based on what you have written.
Difficult to say as stomach pain, nausea and back pain are general symptoms that can be caused by many different conditions.
It is possible to get pregnant if you have sex during your period, the chance is lower but could still happen if you have sex towards the end of your period and you ovulate a few days later.
Most of the urine pregnancy tests can show a positive result as early as 7-10 days after you become pregnant (conception), but to rule out pregnancy you need to do a pregnancy test 7 days after your missed period.
I would recommend that you go see a health care provider about your symptoms. They would be able to do a pregnancy test for you and discuss your symptoms. I am also unsure of your situation but if you are concerned a sexually transmitted infection (STI) could have passed, it would be worthwhile getting tested for this as well.
Also have a look at our web page “what to do if we didn’t use protection” it provides some additional information and resources, e.g. preventing pregnancy by using the emergency contraceptive pill.
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