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Your symptoms of lower abdominal pain, breast soreness, spotting, lighter periods, lower backpain could be suggestive of pregnancy. Avoid any heavy work and intercourse unless pregnancy is confirmed or ruled out. Go ahead with visit to your doctor and request for a blood HCG level test to confirm/rule out pregnancy. Urine pregnancy tests cannot be relied upon for accuracy as they have high incidence of showing false negative results (a negative result even though you are pregnant). look out for any other new symptoms development like increased sleep, hunger and tiredness, mood changes, headaches, fatigue, nausea, constipation, nipple sensitivity, blue veins on breasts, increased frequency of urine, pelvic discomfort and heaviness, food cravings, dizziness, increase in vaginal discharge etc.
Take care.