one test was invalid and the other 2 were negative but...

Hi,
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The intercourse if closer to your ovulation time, could lead to pregnancy as suggested by your symptoms (abdominal cramps, spotting, nausea, low back pain, tender breasts, darker nipples, fatigue, mood swings). If fertile mucus had been present, sperms can stay alive for 5-7 days, and fertilize the egg after ovulation (egg has only 24-48 hours to get fertilized). The symptoms before expected period time could suggest probable implantation. Wait and watch for your periods. Look out for development of any other new symptoms if pregnancy is suspected. If periods have not started within 10 days of expected period dates, or if you have got abnormal periods, or if you suspect chances of pregnancy, consider visiting your doctor/gynecologist at the earliest for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). You may also consider taking a repeat urine pregnancy test (on an early morning first urine sample) after 7-10 days of missed/altered periods to see if you get positive results, or get it confirmed by a blood HCG test. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Drink plenty of water. Take adequate rest. Maintain healthy diet.
Take care.