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An initially positive pregnancy test that later becomes negative can happen with very early miscarriages. Such miscarriages can occur with little or no initial blood loss. These are termed chemical pregnancies, as they are lost before they attach to the womb and before they grow big enough to see on ultrasound. Only the pregnancy hormone is detected.
The fact that you have still not had your menses needs to be investigated. Although it may be normal to have a delayed return of ovulation and menses after a miscarriage. The same pregnancy hormone detected in the urine pregnancy test can be done by a blood test for greater accuracy.
An ultrasound may also be beneficial to identify any other cause for your missed period and your abdominal pain. Did you have a history of irregular periods before this episode?
For more information consult an obstetrician and gynaecologist online --> https://www.icliniq.com/ask-a-doctor-online/obstetrician-and-gynaecologist